Optical Distance Measuring Device and Method for Optical Distance Measurement

ABSTRACT

The present invention describes an optical distance measuring device having a pulsed radiation source that is implemented to transmit, in a temporally contiguous radiation pulse period, a radiation pulse having a pulse duration t p  that is shorter than the radiation pulse period, and to transmit no radiation pulse in a temporally contiguous dark period. Further, the optical distance measuring device includes a detector for detecting different amounts of radiation in two overlapping detection periods during the radiation pulse period to capture reflections of the radiation pulse at an object surface and a background radiation and/or in two overlapping detection periods during the dark period to capture background radiation. The optical distance measuring device further includes an evaluator determining a signal depending on a distance of the optical distance measuring device to an object based on the detected amount of radiation. Further, the present invention provides a method for optical distance measurement and for multiple sampling.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a U.S. National Phase entry of PCT/EP2009/002570filed Apr. 7, 2009, and claims priority to German Patent Application No.10 2008 018718.6 filed Apr. 14, 2008, each of which is incorporatedherein by references hereto.

BACKGROUND OF THE INVENTION

The present invention relates to an optical distance measuring deviceand a method for optical distance measurement, as used, for example, in3D cameras.

The technical field of application of the present invention is manifold,as will be described below. Complementary metal oxide semiconductor(CMOS) image sensor technology provides effective options for recordingmeasurement signals in real time at high speed. This is of greatimportance when capturing three-dimensional (3D) distance images. Pulseruntime methods and methods with continuously modulated light serve herefor contactless depth detection. For this, the residual intensity of aninfrared laser light reflected by an object is measured. This is alsoreferred to as 3D distance measurement.

Typical applications where optical distance measurement can be used are,for example, three-dimensional inspection/positioning systems,one-dimensional positioning systems, such as high-rack warehouses orfilling systems, in the automotive field: systems for automobileinterior surveillance, for airbag control, anti-theft systems, lanedetection systems, so-called pre-crash sensor technology systems,pedestrian protection or parking assist systems. It is also possible touse optical distance measurement for topographical survey or for thedetection of persons or for presence sensor technology. Further fieldsof application are traffic monitoring/counting, logistics, industrialautomation or monitoring of different (danger) areas.

In particular in intelligent airbag release and lane detection, highreliability requirements exist for the distance measurement system. Inintelligent airbag control systems, for example, the task of releasingthe airbag with delayed intensity has to be solved in dependence on thedistance of the passenger. Lane detection has to operate reliably alsoin fog, darkness, bad weather conditions and extreme situations withoncoming light. This is possible with 3D CMOS image sensors. Since, dueto the expected legal pressure, there is or will be a high demand forsuch intelligent systems on part of the automotive industry, asignificant market potential results for this field of application.

The advantageous usage of active lighting in three-dimensional (3D) CMOScameras for capturing a three-dimensional distance image that can beused, for example, in the automotive field is described in patents DE19833207 A1, EP 104366 B1 and WO2007/031102 A1.

Existing 3D CMOS image sensors for distance or depth measurement arelargely based on the functional principle of an active image pointsensor or an active pixel sensor (APS). Here, the temporal opening of anexposure window of the pixel is synchronized with the pulsed release ofactive scene lighting. With the desired pulse light for active scenelighting, however, a portion of the unwanted background light is alsodetected. Additionally, the reflectivity of the objects of the scenealso influences the portion of the reflected light. Depending on thedistance of the object, these factors corrupt the payload signal, partlyto a considerable extent. In order to obtain sufficiently exact distanceinformation, several images are captured with the laser (pulse light)turned on or off, as well as with two different effective exposure orshutter times. This approach has several disadvantages. On the one hand,capturing the series image sequences (serial capturing) limits thebandwidth, which is why 3D applications are not possible in high andhighest speed applications. Further, by serial reflectance correction,it is necessitated to pulse the laser source twice, which meansunnecessary energy doubling, which can collide with requirementsregarding eye safety with respect to laser radiation in certain fieldsof application, such as automobile exterior surveillance.

SUMMARY

According to an embodiment, an optical distance measuring device mayhave: a pulsed radiation source implemented to transmit, in a temporallycontiguous radiation pulse period, a radiation pulse having a pulseduration that is shorter than the radiation pulse period, and totransmit no radiation pulse in a temporally contiguous dark period; adetection means for detecting different amounts of radiation in twooverlapping detection periods during the radiation pulse period tocapture reflections of the radiation pulse at the object surface and abackground radiation and/or in two overlapping detection periods duringthe dark period to capture a background radiation; and an evaluationmeans determining a signal depending on a distance of the opticaldistance measuring device to an object, based on the detected amounts ofradiation.

According to another embodiment, an apparatus may have: a capacitivepixel sensor element subjected to a charge or discharge process independence on a measured quantity that can be detected at a pixel sensorelement output; at least one buffer amplifier; first and second samplecapacitances; first and second switches, via which the first or thesecond sample capacitance can be connected to the pixel sensor elementoutput via the at least one buffer amplifier; a controller that isimplemented to control the first and second switches such that the firstswitch is closed in a first time window and the second switch is closedin a second time window, wherein the first time window and the secondtime window temporally overlap such that, at the end of the first andsecond time windows, different voltage signals describing the charge ordischarge process of the capacitive pixel sensor element are applied tothe first and second sample capacitances.

Another embodiment may have a double sampling system having a pluralityof inventive apparatuses, wherein the apparatuses are arranged inarrays, wherein an evaluation means that can be coupled, with the helpof a controller for controlling the arrays or the individual apparatusesvia a select switch and a read line, to the voltage signals of thesample capacitances describing the charge or discharge process of thecapacitive pixel sensor element is associated with every array or everysingle apparatus.

According to another embodiment, a method for optical distancemeasurement may have the steps of: emitting a radiation pulse with apulsed radiation source implemented to transmit, in a temporallycontiguous radiation pulse period, a radiation pulse having a pulseduration t_(p) that is shorter than the radiation pulse period, and totransmit no radiation pulse in a temporally contiguous dark period;detecting different amounts of radiation with a detection means that isimplemented to capture reflections of the radiation pulse at an objectsurface and background radiation in two overlapping detection periodsduring the radiation pulse period and/or to capture background radiationin two overlapping detection periods during the dark period; anddetermining a signal depending on the distance to be measured based onthe detected amounts of radiation.

According to another embodiment, a multiple sampling method may have thesteps of: electrically coupling a first sample capacitance to an outputof a capacitive pixel sensor element via a buffer amplifier during afirst time window having a first duration, and electrically coupling asecond sample capacitance to the output of the capacitive pixel sensorelement via a buffer amplifier during a second time window having asecond duration, wherein the first and the second time window temporallyoverlap, such that at the end of the first and second time windowsdifferent voltage signals describing a charge or discharge process ofthe capacitive pixel sensor element are applied to the first and secondsample capacitances.

Another embodiment may have a computer program having a program code forperforming the inventive method, wherein the program code is performedon a computer, a microcontroller or a digital signal processor.

The object solved by the invention consists of the generation of anoptical distance measuring device or an apparatus and a method foroptical distance measurement, which can detect both the extraneousbackground light components, and the reflectance components in a singlemeasurement cycle. This means pulsing the radiation or laser sourcetwice per 3D picture can be avoided. The problem of eye safety withregard to laser energy is significantly reduced, since only half of theoriginally used laser energy is needed. Further, by omitting a secondpulse sequence, maintaining a waiting time for recovery of the laser oftypically several microseconds is simplified, since the wholemeasurement cycle per 3D picture or 3D distance measurement can bereduced to merely one laser pulse sequence. Advantageously, compared toseries image capturing, this reduces the measurement time by half in thefirst approximation.

An optical distance measuring device according to the present inventioncomprises a pulsed radiation source implemented to transmit, in atemporally contiguous radiation pulse period, a radiation pulse having apulse duration tp that is shorter than the radiation pulse period, or tobe activated, and to transmit no radiation pulse or to be deactivated ina temporally contiguous dark period. Further, the optical distancemeasuring device according to the present invention comprises adetection means for detecting different amounts of radiation in twooverlapping detection periods, during the radiation pulse period tocapture reflections of the radiation pulse at an object surface andbackground radiation, and/or during the dark period to capturebackground radiation. Additionally, the optical distance measuringdevice comprises an evaluation means determining, based on the detectedamounts of radiation, a signal depending on a distance of the opticaldistance measuring device to an object.

Further, the present invention provides a method for optical distancemeasuring by emitting a radiation pulse with a pulsed radiation sourceimplemented to transmit, in a temporally contiguous radiation pulseperiod, a radiation pulse having a pulse duration tp that is shorterthan the radiation pulse period, and to transmit no radiation pulse in atemporally contiguous dark period. Further, the method comprisesdetecting different amounts of radiation with a detection meansimplemented to capture reflections of the radiation pulse at an objectsurface and background radiation, in two overlapping detection periodsduring the radiation pulse period, and/or to capture a backgroundradiation during the dark period, and further determining signalsdepending on the distance to be measured based on the detected amountsof radiation.

Embodiments of the present invention provide the advantage that adetection means for detecting different amounts of radiation in twooverlapping detection periods is implemented in parallel, so that theinformation on distance, reflectance as well as background portionscontained in a reflected radiation pulse can be captured in a singleradiation pulse period, and a background signal portion without areflected radiation pulse can be detected in a second dark period. Then,in an evaluation means implemented in parallel in one embodiment, asignal depending on a distance of the optical distance measuring deviceto an object can be determined based on the detected radiation energies.

Hence, the inventive optical distance measuring device or the inventivemethod for optical distance measurement can provide the possibility ofdisproportionately increasing the measuring speed and to reduceradiation energy necessitated for measuring by half.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be detailed subsequentlyreferring to the appended drawings, in which:

FIG. 1 is a schematical illustration of the measuring arrangement fordepth or distance detection;

FIG. 2 is the circuit diagram of an analog readout path for a 3D camera;

FIG. 3 a is the time scheme for distance measurement with extraneouslight correction for a circuit according to FIG. 2;

FIG. 3 b is an illustration of the connection between pixel reset andshutter signal of FIG. 3 a;

FIG. 4 is a schematical illustration of an optical distance measuringdevice according to an embodiment of the present invention;

FIG. 5 is a circuit for an optical distance measuring device or anapparatus for distance measurement with double sampling according to anembodiment of the present invention;

FIG. 6 is a clocking scheme for illustrating the mode of operation ofthe apparatus for distance measurement with double sampling according toFIG. 5;

FIG. 7 is a simulated time diagram for illustrating the mode ofoperation of embodiments of the present invention;

FIG. 8 is a circuit for an optical distance measuring device or anapparatus for distance measurement with double sampling according to afurther embodiment of the present invention;

FIG. 9 is a further circuit for an optical distance measuring device oran apparatus for distance measurement with double sampling according toa further embodiment of the present invention;

FIG. 10 is a schematic voltage time diagram for illustrating theextrapolation of the background light portion with the help of thecircuit illustrated in FIG. 9 and FIG. 8;

FIG. 11 a is a further schematic voltage time diagram for illustratingthe extrapolation of the background light portion with the help of thecircuit illustrated in FIG. 9 and FIG. 8;

FIG. 11 b is a schematic illustration regarding the reset phases for anapparatus according to embodiments of the present invention;

FIG. 12 is a block diagram of a double sampling measurement systemaccording to an embodiment of the present invention having a pluralityof distance measuring devices arranged in a matrix; and

FIG. 13 is a flow diagram of the method for optical distance measurementaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Regarding the following description of the embodiments of the presentinvention, it should be noted that, in order to simplify matters, thesame reference numerals are used in the entire description in thedifferent figures for functionally identical or equally acting, orfunctionally equal, equivalent elements.

FIG. 1 illustrates a schematical arrangement of a measurement system 100for optical distance measurement and a possible pixel architecture 125 aof an image sensor. For obtaining distance or depth information, atarget object 105 is irradiated with a pulsed laser 100. In aphase-locked manner thereto, the exposure of an integratingphotoreceiver array in the image sensor 125 begins. The distance d ofthe sensor to the irradiated object 105 is decisive for the runtime ofthe light pulse 110 a emitted by the laser source 110 and hence for theoptical power measured by the receiver 125 in a fixed exposure timewindow. The optical distance measuring device 100 can, for example, haverefractive optics 102 and receiving optics 104, mapping the laser pulseto the target object or the reflected beam to the receiver 125. Afterthe refractive optics 102, the laser pulse can have an opening angle θ.The pulse light source or laser light source 110 can have a negligiblysmall angle α with regard to the receiver 125, which is why the wholedistance traveled by the emitted and reflected light pulse is 2d. With aknown fixed angle α and a known distance d, this local difference canalso be compensated in a computational manner. The laser pulse isreflected at the irradiated surface 105, wherein the object surface canbe a Lambert radiator, and then impinges onto the photosensitive surfaceof the receiver unit 125 after a runtime delay t_(run)=2d/c_(light) withthe residual energy E_(laser). The receiver unit can, for example, be aCMOS image sensor. The laser pulse from the pulse light source 110 andthe exposure measurement in the receiver start in an adjustablephase-locked or time-locked manner, i.e. in this embodiment, forexample, simultaneously at a time t=T_(1,0) (see FIG. 3 a). Thus,depending on the size of the distance d, the runtime until the reflectedpulse is detected in the receiver 125 will be shorter or longer, and theperiod 303 shifts along a time axis t in FIG. 3 a to the left or to theright. With increasing distance, the delay time increases and the periodt_(run) shifts to the right. In the receiver 125, the measurement istaken in an integrative manner in a short time interval t_(p) or in alonger time interval t_(s), the so-called shutter time. In thisarrangement, the signal measured by the photoreceiver 125 is linearlyproportional to the distance. As illustrated schematically in FIG. 1,the photoreceiver 125 can consist of a series of pixels 125 a. The pixel125 a can, for example, be a PN photodiode 130. The PN photodiode 130can generate a photocurrent or a photo charge by lighting.

The pixel 125 a can, for example, be an N well/P substrate photodiodewith a parallel barrier layer capacitance C_(D) 132, which can beconnected to ground 133 with one terminal, and connected to a bufferamplifier 134, which can be implemented as a voltage follower (sourcefollower), with a further terminal.

Thus, the receiver 125 can, for example, be a sensor made up of activepixel elements (active pixel sensor (APS)), which is, for example,produced in CMOS technology and is frequently referred to as a CMOSsensor. By using CMOS technology, it is possible to integrate furtherfunctions, such as free selection of the sensor area to be read out,exposure control, contrast correction or analog/digital conversion.

It is also possible that the pixel 125 a is a passive pixel, where thepixel has no amplifier 134 amplifying or rendering the signal created bythe photodiode. Exemplarily, two principles for pixel readout arementioned here. In a first principle of charge readout, a chargeaccumulated on a barrier layer capacitance 133, which is generated dueto the internal photo effect during the interaction of a radiation withthe photodiode, is read out.

In another pixel principle, the pixel has a buffer amplifier in the formof a voltage follower 134. In pixels operating according to thisprinciple, a voltage applied across the pixel is read out. Thereby, tvoltage is thereby reduced by the charges of the electrons and holesgenerated because of the inner photo effect.

In the following, the mode of operation of a receiver 125 will beillustrated exemplarily based on a circuit (FIG. 2) for a pixel 125 a ofthe image sensor 125 and an associated time diagram (FIG. 3 a)illustrating the measurement regulation for distance measurement withextraneous light correction.

FIG. 2 shows the schematical structure of a standard 3D runtime analogreadout path for a pixel 125 a. The pixel 125 a in FIG. 2 comprises, forexample, an N well/P substrate photodiode PD 130 with the barrier layercapacitance C_(D) 132 as a detection unit. At the beginning of everyexposure process, the photodiode PD 130 is biased with a reset potentialV_(reset) 229 via a reset switch 227 that can be implemented astransistor Q1. Additionally, the pixel 125 a has a buffer amplifier 134,for example a so-called source follower or voltage follower. Normally,the buffer amplifier 134 is implemented as a source follower and buffersthe voltage applied across the capacitance C_(D) 132. The output of thesource follower 134 can be connected to a sample and hold circuit 250via a switch or shutter 252 with a sample capacitance C_(S) 254 and afurther buffer amplifier 264 of the sample and hold circuit 250. Theoutput of the buffer amplifier 264 can be connected to a subsequentcircuit for (correlated) double sampling—(c)DS circuit 270 via a selecttransistor 255.

At the beginning of an exposure process, the photodiode 130 and hencethe barrier layer capacitance C_(D) 132 and the sample capacitance C_(S)254 is charged to the defined start potential U_(ref) 220 after closingthe reset transistor Q₁ 227 and the switch 252. When subsequently thereset switch 227 is opened and hence the photodiode 130 with the barrierlayer capacitance 132 is decoupled from the voltage U_(ref) 229, theelectron hole pairs resulting from the radiation impinging on thephotodiode discharge the capacitances C_(D) 132 and C_(S) for so longuntil the shutter 252 is deactivated, i.e. opened. A voltage reproducingthe temporal discharge process by the impinging radiation is hencetransmitted to the capacitance C_(S) 254 via the buffer amplifier 134.After an integration time τ_(int), this process is interrupted. Thismeans after opening the switch 252, the sample capacitance 254 of thesample and hold circuit 250 has a residual voltage U₁ describing thedischarge process of the barrier layer capacitance of the photodiode dueto the incident radiation in a period. Spatial distance detection ordistance measurement becomes possible by arranging the pixels in amatrix (array) and by means of synchronous exposure of all receivingelements (pixels) and the shutter signal at the photodiode. Opening theshutter, i.e. terminating the integration, interrupts the current flowto the sample capacitance C_(S) 254, so that the current value of thelight-proportional elementary charges accumulated onto the capacitancesC_(D) 232 and C_(S) 254 is “frozen”. If the reflected laser pulse andthe integration time overlap partly during the shutter signal, thevoltage U₁ applied to C_(S) 254 will include information on the distanced and on extraneous light portions. If the duration of the shuttersignal and hence the integration time is larger than the pulse widtht_(p) of the laser, the voltage U₂ applied to the sample capacitance 254is a measure for the completely accumulated laser power density, furtherhaving information on extraneous light portions, reflectance portions rand sensitivity R of the used photodiode. This circumstance is used forcompensating extraneous light or background light portions. Themeasurement result obtained by integration in a time window partlyoverlapping with the reflected laser pulse includes, in addition to thedistance d and the extraneous light portions, information on thereflectance r of the exposed object, the sensitivity R of the usedphotodiode which are created by inevitable influences of environmentallighting.

For obtaining the pure distance value, i.e. for distance measurement, inthe conventional method, two measurement cycles are performed one afterthe other, which differ in the integration time or the duration of theshutter signal, i.e. the time in which the shutter or the switch 252 isclosed and the discharge process of the barrier layer capacitance 232and the sample capacitance 254 is performed by light incident on thephotodiode. A first integration window with a first integration timecorresponding to the shutter signal can therefore correspond to theduration and the pulse width t_(p) of the laser pulse used for distancemeasurement, and a second integration time t_(s) or t_(int2) of a secondintegration window can be larger than the pulse width t_(p) of the laserpulse.

The time scheme or measurement regulation for distance measurement withthe circuit illustrated in FIG. 2 is illustrated schematically in thetime diagrams of FIG. 3 a. The x axis 301 of diagrams a-f in FIG. 3 arepresents time axes t, on which the timeline of the interaction ofindividual components of the circuit for distance measurement withextraneous light correction is illustrated. Both the time axes t 301 andthe respective y axes 302 a-302 f are illustrated in arbitrary units andserve merely for illustrating the time events. On the y axis in FIG. 3a, the following is illustrated: in diagram a the laser pulse, indiagram b the reflected laser pulse, in diagram c the switching behaviorof the pixel shutter, in diagram d the voltage at the samplecapacitance, in diagram e the select signal and in diagram f the resetsignal.

As illustrated in FIG. 3 a, in a first cycle I 360 comprising twosub-cycles I_(A) 306 and I_(B) 308 and that can last, for example,20-2000 nanoseconds, a photodiode is biased in a defined manner inresponse to a reset signal 334 (diagram f), as described above, i.e. abiased barrier layer capacitance is build up. When resetting the pixelby the switch 227 (FIG. 2), the switch 252, i.e. the shutter, can beclosed simultaneously. The integration time starts when the reset is on“0”, i.e. the reset switch 227 is opened again, and stops when theshutter signal is subsequently on “0”, i.e. the switch 252 is openedagain.

FIG. 3 b shows this temporal connection between the shutter signalillustrated in diagram c in FIG. 3 a and the pixel residual signal asillustrated in diagram fin more detail. During the pixel reset 334, 336,338 and 340, the pixel shutter 314, 312, 316 and 317 should be closed,as is illustrated in FIG. 3 a. This means that the pixel reset and theshutter signal start simultaneously and in an overlapping manner.However, the integration time t_(int) starts only after the pixel resetis terminated, i.e. is again set to “0”. The integration stops when theshutter or the switch 252 is opened, i.e. the shutter signal returns to“0”.

During the reset or after the reset, a laser pulse 304 (diagram a, FIG.3 a) having a pulse duration t_(p) 304 a is emitted from a laser source.Simultaneously with the reset of the pixel, the pixel shutter 252, asdescribed in FIG. 2, is closed for a duration 314 corresponding at leastto the duration of the laser pulse 304. This is illustrated in diagramc. During the first time window 314, depending on the detectedextraneous light portion and the detected reflected laser pulse, thesample capacitance 254 is discharged and hence the voltage U₁ across thesample capacitance 254 is reduced, as is illustrated in diagram d. Thelaser pulse 304 reflected by an object (FIG. 3 a, diagram b) with itspulse width 304 a necessitates a certain runtime to the object and backfrom the object to the sensor. It follows that the reflected laser pulse312 is shifted to the right by the runtime t_(run) 303 on the time axis301 in relation to the pulse window 304. As is illustrated in diagrams cand d, the pixel detects only extraneous light portions during theruntime t_(run) 303 of the laser pulse, which results in a voltagesignal U_(1,extraneous) 320 at the sample capacitance 254. In a secondtime range t_(p)-t_(run) of the integration window 314, a superposedvoltage U₁ results, which includes both the extraneous light portion andthe laser light portion reflected by the object. The second time portionof the integration window 314 (with t_(int)≧t_(p)) corresponds to theoverlapping period of the reflected laser pulse 312 with the firstintegration window of the pixel shutter 314. This means, for thisexample, this corresponds to a duration t_(p)-t_(run), which includes,apart from the extraneous light or background light portions, thereflectance and the sensitivity, also the distance information to theobject. As shown in diagram d, the following applies for the voltage U₁:U₁˜E_(laser) (t_(p)-t_(run))+E_(extraneous) t_(p). By closing 326 theselect switch 255 (see FIG. 2), the voltage signal U₁ is passed on tothe circuit for (correlated) double sampling 270) this circuit for(correlated) double sampling and its mode of operation will be discussedin more detail below.

By a further reset pulse 336, as is illustrated in diagram f, a firstsub-cycle I_(A) 306 is terminated during the first cycle 360 and thesecond sub-cycle I_(B) 308 where no laser pulse is emitted begins. Inthis second sub-cycle I_(B) 308, integration is performed for a durationt_(int)≧t_(p), which corresponds at least to the pulse period 304. Forthis purpose, the pixel switch is closed again, and an extraneous lightportion 320 without reflection portions is integrated by the laser pulseor charges are accumulated in a time window 315 having the durationt_(int)≧t_(p). This means that, at the end of the integration time, thepixel switch 252 is opened so that a voltage U_(1,extraneous) 320corresponding to the background or extraneous light portion is appliedto the sample capacitance 254 (FIG. 2). The voltage applied to thesample capacitance 254 can then be passed on to the circuit for(correlated) double sampling 270 by closing the select switch 255. Bythis, the first cycle I 360 consisting of the first sub-cycle I_(A) 306and the second sub-cycle I_(B) 308 is terminated. In order to be able toperform correct correction with regard to reflectance anddistance-induced attenuation of the laser pulse for the distancemeasurement, a second cycle II 370 follows, which is also divided intotwo sub-cycles II_(A) 307 and II_(B) 311. The second cycle II 370 can,for example, again comprise a period of 20 to 2000 nanoseconds. Analogto the description of the first cycle I 360, in response to a resetsignal the pixel with the sample capacitance 254 is biased to a definedreference voltage. For example, as is illustrated in FIG. 3 a, a secondlaser pulse 305 with the laser pulse period 304 a t_(p) can be emittedsimultaneously to the second time window 316 for integrating thephotodiode signal. The second time window 316 has a duration t_(s) ort_(int2) 316 a, which is larger than the pulse width or duration 304 a(t_(p)) of the laser pulse 304. As is illustrated in diagram b in FIG.3, analog to the first cycle 360, the reflected laser pulse 312 willimpinge on the pixel sensor with its pulse width t_(p) offset by theruntime t_(run). As is illustrated in diagram c in FIG. 3, in the secondcycle II 370, the second integration time window 316 has a durationt_(s) 316 a that is longer than the duration t_(p) of the laser pulse.For this reason, the photodiode signal is integrated for the whole laserpulse t_(p) and the respective extraneous light portion. This isillustrated in the voltage curve 322 in diagram d. A voltage U₂ isapplied to the sample capacitance 254, for which the following applies:U₂˜(E_(laser) t_(p)+E_(extraneous) t_(s)). It follows that the voltageU₂ includes both extraneous light portions and the completely integratedreflected laser pulse power. The voltage U₂ applied to the samplecapacitance 254 after opening the shutter or closing the time window 316can then again be passed on to the circuit for (correlated) doublesampling 270 by closing 330 the select switch 254. Thereby, the firstsub-cycle II_(A) 307 of the second cycle II 370 is terminated. Byresetting 340 the photodiode by closing a reset switch 227 (FIG. 2), thepixel and therewith the barrier layer capacitance can be reset to apredefined starting value. The reset 340 of the pixel with the switch227 can again be performed simultaneously with closing the shutterswitch 252. The integration time of the time window 316 starts againwhen the reset is on “0”, i.e. the reset switch 227 is opened and stopswhen the shutter signal is on “0” after wards, i.e. the switch 252 isopened again. In the second sub-cycle II_(B) 311 of the second cycle II,also no laser pulse is emitted. But, again, integration is performedover a second period 317 having a duration t_(s) 316 a, which is givenby opening the reset switch 227 and the period where the shutter switch252 is closed. Thereby, the background light or the extraneous lightportion at the signal U₂ is integrated in a time window identical to thefirst sub-cycle 308, but without reflected laser pulse. This means thevoltage U_(2,extraneous) 324 applied to the sample capacitance 254includes or corresponds to only the extraneous light portion of theaccumulated charge or voltage at the capacitance. After opening thepixel shutter 252 or closing the time window 317, then, for exampleagain by controlling 332 the select switch 255 (FIG. 2), the voltagesignal U_(2,extraneous) 324 can be transferred to the circuit for(correlated) double sampling 270. A continuous transfer of the voltagesignal during the integration time t_(int) to the circuit for(correlated) double sampling is also possible.

As is shown in FIG. 2, the circuit for (correlated) double sampling(CDS) comprises an operational amplifier 280, wherein the output 280 aof the operational amplifier can be connected to the “non-inverting”input 280 b of the operational amplifier 280 via a reset switch 282. The“inverting” input 280 c of the operational amplifier 280 can be on anoffset potential 275. The correlated double sampling circuit 270 can beconnected to the sampling capacitance C_(S) 254, which can be connectedto ground 133 on one terminal side, and the buffer amplifier 264 via theselect switch 255. The buffer amplifier 264 can have, for example, anamplification of 1.

Further, the circuit for (correlated) double sampling 270 has a samplecapacitance C_(C1) 274 and a hold capacitance C_(H) 276, which isconnected to ground 133 on one terminal side. The sample capacitanceC_(c1) 274 is connected on one side with C_(H) and on its other terminalside with the “non-inverting” input 280 b of the operational amplifierand with a feedback capacitor C_(F) 284. The feedback capacitor 284 canbe connected either with the output 280 a of the operational amplifiervia a feedback switch Φ₂ 288 or with a reference potential 275 via areference potential switch Φ₅ 290. An output voltage of the correlateddouble sample circuit 270 can then be applied to the output 299.

The functional principle of the circuit for (correlated) double sampling270 is based on the fact that a voltage difference, which is applied tothe output 299, can be formed between two voltage values, namely a firstvoltage value V1 and a second voltage value V2. The first voltage valueV1 can be, for example, the reset voltage value 275, and the secondvoltage value V2 can be, for example, a signal voltage value. The firstvoltage value V1 and the second voltage value V2 are sampled at twosubsequent times shifted by one clock phase. Thereby, a voltage value isat first temporarily stored on the sample capacitance 254, 274, so thatin a second clock phase, a differential signal or a differential voltagecan form between the two voltage signals at the feedback capacitance284, which is on the reset voltage value 275. Thereby, a possiblyexisting offset and a low-frequency noise that can occur due to theamplifier 280 or the preceding buffer amplifier or sample and holdcircuit 250 or the pixel element 125 a, and which can have an effect onboth voltage values (the first voltage value V1 and the second voltagevalue V2), is suppressed. Additionally, the correlated double samplecircuit 270 allows a low-frequency noise suppression compared to the CDSfrequency.

In the following, the mode of operation of the correlated double samplecircuit 270 will be discussed in more detail. In a first phase, wherethe switch 282 Φ₃ is closed, the input 280 b and the output 280 a of theoperational amplifier 280 are short-circuited via switch 282. The switchΦ₅ 290 is open and only closed prior to the integration phases 306, 308,307 and 311 (FIG. 3 a) or prior to all multiple exposures for biasingonce to a reference voltage 275. The switch Φ₂ 288 is open, i.e.non-conductive. During this phase, the feedback capacitor 284 is chargedup to an offset voltage of the operational amplifier 280. The samplecapacitance 274 and the hold capacitance 276 are charged up to adifferential voltage applied to the CDS circuit 270 after closing theselect transistor 255 and corresponds to the reference voltage 275 minusthe operational amplifier offset voltage. This first time phasecorresponds to the first sub-cycle 306 in FIG. 3 a. During a secondphase, the switch 288 Φ₂ is closed and the switch Φ₃ 282 is opened.Thereby, the operational amplifier 280 is switched to its feedbackamplification. After the second sub-cycle 308, the voltage 320U_(1,extraneous) is applied to the input of the correlated double samplecircuit 270, and the difference of voltagesU_(1,laser,extraneous)−U_(1,extraneous) plus a reference voltage 275 isapplied to the output 280 a of the operational amplifier. Analogously,in subsequent clocks, the differential voltageU_(2,diff)=U_(2,laser,extraneous)−U_(2,extraneous) plus the referencevoltage 275 is provided at the output 299 of the correlated doublesample circuit 270.

For obtaining the pure distance value, in previous methods, twomeasurement cycles 360, 370 are performed one after the other, whichdiffer, for example, in the duration of a shutter signal t_(int), i.e.for example t_(p) or t_(s), in a first measurement cycle, the shutterduration or the time window for the integration is larger or equal tothe duration of the laser pulse and at the same time partly overlappingwith the reflected laser pulse. Thereby, apart from the background lightportion or the extraneous light portion, the leakage current portion ofthe photodiode and the reflectance, the distance information included inthe voltage U₁ is also detected. Every cycle is divided again into asub-cycle I_(a) 306 and I_(b) 308. In the first sub-cycle 306, the lasersource is switched on, so that the voltage U_(1,laser,extraneous)applied to the output of the pixel can be described by the followingequation 1:

U _(1,laser,extraneous) =R·r[E _(laser)·(t _(int) −t _(run))+E_(extraneous) ·t _(int)]  (1)

In equation 1, R corresponds to the sensitivity of the photodiode, r tothe reflectance of the exposed object, E_(laser) to the power/radiationstrength of the laser pulse, E_(extraneous) to the extraneous opticalpower/radiation strength and t_(int) to the integration time t_(int),which corresponds, in this example, to the laser pulse duration t_(p)and t_(run) to the runtime of the laser pulse. Generally, t_(int)≧t_(p)applies.

Equation 1 includes a runtime-dependent portion and a portion dependingon extraneous light. In the sub-cycle 308, the measurement is repeated,but without laser light pulse. The following applies:

U _(1,extraneous) =R·r·E _(extraneous) ·t _(int)  (2)

In equation 2, U_(1,extraneous) includes merely the extraneous light orbackground light portion.

In CMOS technology, it is possible to subtract analog signals directlyon the image sensor chip with very high accuracy, and this is normallyperformed in a so-called circuit for correlated double sampling (CDSstage), as described above. Subtraction of the two voltages with thehelp of the CDS stage as described above provides:

U _(1,diff) =U _(1,laser,extraneous) −U _(1,extraneous) =R·r·E _(laser)(t _(int) −t _(run))  (3)

Since the measurements in the sub-cycles 306 and 308 are very close toeach other in time, the extraneous light portions are correlated atalmost 100%, which is why equation 3 is valid in the illustrated form.However, equation 3 also includes the reflectance r, which can varysignificantly from pixel to pixel—in the extreme case up to a factor1:40 for the reflection—and the sensitivity R of the photodiode, whichcan also vary from pixel to pixel for fabrication reasons. Theseparameters can now be compensated in the second measurement cycle 370.Beforehand, however, the differential value U_(1,diff) is stored in thecamera system, since the pixel and the CDS circuit are reset for thenext measurement cycle. The following second measurement cycle II 370 isidentical to the measurement cycle I 360 except for the length of theshutter signal. The second shutter time window 316 has now asignificantly longer duration t_(s) 316 a than the duration of the laserpulse t_(p), so that the whole pulse form of the reflected laser pulsein the pixel is integrated during the second time window. For thatreason, no information on the distance d to the object to be measured iscontained in the voltage signal U₂. This information is obtained by“cutting off” the received laser pulse, as was described in connectionwith the first cycle. For the sub-measurement cycle II_(A) 308 of thesecond cycle 370, the following equation applies:

U _(2,laser,extraneous) =R·r(E _(laser) ·t _(int) +E _(extraneous) ·t₂)  (4)

As in the cycle I_(A) 306, a laser and an extraneous light or backgroundportion is captured. The second sub-cycle II_(B) 311 again provides thenecessitated extraneous light portion U_(2,extraneous) for compensation:

U _(2,extraneous) =R·r·E _(extraneous) ·t _(s)  (5)

In a subsequent voltage subtraction that is performed analogously, aswas described in connection with the first cycle, in the correlateddouble sample circuit (CDS stage) 270, the following equation resultsfor the voltage U_(2,diff):

U _(2,diff) =R·r·E _(laser) ·t _(int)  (6)

In the camera system (not shown in the figures) the quotient fromequation 3 and equation 6 can be calculated. The following applies:

$\begin{matrix}{\frac{U_{1,{diff}}}{U_{2,{diff}}} = \frac{\left( {t_{int} - t_{run}} \right)}{t_{int}}} & (7)\end{matrix}$

With the connection for the runtime t_(run)=2d/c_(light) and theassumption that the laser pulse source as well as the CMOS image sensor,the distance or the spatial position of the laser pulse source and theimage sensor are known and can be compensated in a computational manner,the following applies for the distance d_(x,y) of a pixel (x, y) in apixel array with x by y pixels to its corresponding object point in thescene:

$\begin{matrix}{d_{x,y} = {\frac{c_{light}}{2} \cdot {t_{int}\left( \left( {1 - \frac{U_{1,{diff}}}{U_{2,{diff}}}} \right)_{x,y} \right)}}} & (8)\end{matrix}$

In practice, the sensitivity and hence the distance resolution can beincreased by executing cycles I 360 and II 370 each several times oneafter the other and adding up the differences U_(1,diff) and U_(2,dlff)in the circuit for (correlated) double sampling and storing them in ananalog memory on the chip. Therewith, in N repetition cycles of thecycle I, the distance resolution Δd improves by the factor root (N). Forexplaining the measurement regulation, however, considering a singlecycle is sufficient. Apart from the extraneous light portion and thereflectance, the measurement regulation described herein efficientlycompensates dark current portions and low-frequent noise portionsoriginating from the CMOS sensor, since the same are highly correlatedwith the laser pulse source due to the immediate measurement after theexposure. The great disadvantage of the conventional measurement methoddescribed here is the laser recovery time that has to be kept betweencycles I and II. Further, for the standard method for distancemeasurement described here for explanation purposes, one laser orradiation pulse each is necessitated both in the first cycle I 360 andin the second cycle II 370. The serial reflectance correctionnecessitates pulsing the laser source twice, which means unnecessarylaser energy doubling, which can collide with the requirements regardingeye safety in certain cases of application (automobile externalsurveillance).

An optical distance measuring device 400 according to an embodiment ofthe present invention is illustrated in FIG. 4. The optical distancemeasuring device 400 has a pulsed radiation source 110 which isimplemented to transmit, in a temporally contiguous radiation pulseperiod, a radiation pulse having a pulse duration t_(p) that is shorterthan the radiation pulse period, and to transmit no radiation pulse in atemporally contiguous dark period. Further, the optical distancemeasuring device 400 has a detection means 420 for detecting differentamounts of radiation in two overlapping detection periods, during theradiation pulse period to capture reflections of the radiation pulse atan object surface and a background radiation and/or in two overlappingperiods, during the dark period, to capture background radiation.Further, the optical distance measuring device 400 has an evaluationmeans 470, which determines a signal depending on a distance of theoptical distance measuring device to an object 105 based on the detectedamount of radiation. A receiver 425 can comprise one or severaldetection means 420 and evaluation means 470.

The pulsed radiation source can, for example, be a pulsed laser emittinga radiation pulse in the ultraviolet (UV), visible or infrared (IR)spectral range. The emitted radiation pulse can also be a modulatedradiation pulse. In this case, an object whose distance is to bedetermined will be irradiated with modulated light, wherein the signalreceived by the pixel will be demodulated on the receiver side, suchthat the phase difference between the transmitted and the reflectedsignal provides the information on the distance to the object.

The detection means 420 can be implemented, for example, such that thetwo overlapping detection periods during the radiation pulse periodstart synchronously with the radiation pulse of the pulsed radiationsource, wherein a first detection period with a duration t_(int)corresponds, for example, to the pulse duration t_(p), and a seconddetection period has a duration t_(int2) that is longer than theradiation pulse duration t_(p) and/or that the two overlapping detectionperiods during the dark period start synchronously after the terminationof the radiation pulse period, wherein a third detection periodcorresponds, for example, to the pulse duration t_(p) and a fourthdetection period has a duration t_(int2) that is longer than the pulseduration t_(p).

In a different embodiment, the detection means 420 can be implementedsuch that the two overlapping detection periods for detecting thereflections of the radiation pulse define the radiation pulse period andthe detection periods start phase-locked and temporally offset with theradiation pulse of the pulsed radiation source. The first detectionperiod can be smaller, larger or equal to t_(p) (t_(int)≧t_(p)). Thesecond detection period can have a duration t_(int2) that is longer thanthe radiation pulse duration t_(p) (t_(int2)>t_(p)). The two overlappingdetection periods during the dark period can start synchronously or in aphase-locked manner, wherein a first detection period of the dark periodcan have a duration t_(int3) and a second detection period of the darkperiod can have a duration t_(int4). The durations of the first andsecond detection periods in the dark period can be different. Inembodiments, the duration of the first and second detection periods ofthe dark period can correspond to the duration of the first and seconddetection periods of the radiation pulse period. The dark period can bebefore or after a radiation pulse period. The detection periods are alsoreferred to as integration windows having an integration time or as ashutter or shutter signal having a respective shutter or integrationtime.

In embodiments, the detection means 420 can further have an opticalsensor for detecting different amounts of radiation providing a signal,for example an electrical signal, based on a charge or voltage, whichcan then be detected in the overlapping detection periods.

In other embodiments of the present invention, the evaluation means 470of the optical distance measuring device 400 can be implemented, forexample, such that the signal to be determined by the evaluation means470 is a differential signal. These differential signals can begenerated by the evaluation means by subtraction of signals temporarilystored in the evaluation means 470. These temporarily stored signalsdepend on different amounts of radiation detected in two overlappingdetection periods during the radiation pulse period and the dark periodby the detection means 420. In other words, the evaluation means 470 isimplemented such that differential signals are formed by subtractionbetween the two signals detected in the two respective overlappingdetection periods in the radiation pulse period and the dark period,which have a defined dependency on the amount of radiation detected bythe detection means.

FIG. 5 illustrates, in a further embodiment, a detection means 420 andan evaluation means 470 in the form of a circuit diagram. In oneembodiment, the detection means 420 comprises a pixel element 125 a aswell as a sample and hold circuit 550. The pixel element 125 a has aphotodiode 130 and a barrier layer capacitance 132 of the photodiodearranged in parallel, wherein the anode of the photodiode and a terminalof the barrier layer capacitance 132 are connected to a ground potential133. The photodiode and the barrier layer capacitance can be coupled toa reset voltage V_(reset) 229 via a reset switch 227. As is illustratedin FIG. 5, the cathode and the second terminal of the barrier layercapacitance are connected to a buffer amplifier 134. The output of thebuffer amplifier 134, which can, for example, be a voltage follower(source follower) having an amplification of 1, is connected to a sampleand hold circuit 550 constructed in parallel. The sample and holdcircuit 550 has one sample capacitance 254 a and 254 b each in twoparallel branches, wherein one terminal each of the sample capacitances254 a and 254 b is connected to the ground potential 133 and therespective other terminal of the sample capacitances is connected ineach case to the output of the buffer amplifier 134 in every parallelbranch via one switch 252 a and 252 b each. Further, the samplecapacitances 254 a and 254 b are connected to a further switch 560 a and560 b via a circuit node 253 a or 253 b and via a logical AND gate.

In the embodiment, the switches 560 a and 560 b can be controlled by acontrol logic 562 a, 562 b. In this embodiment, opening and closing theswitches 562 a and 562 b depends on a select signal and the Φ_(1a) orΦ_(1b) signal. The sample and hold capacitances 254 a and 254 b can beconnected to a further buffer amplifier 264 via switches 560 a and 560b, and via the same to the select switch 255 and hence the output of thesample and hold circuit 250.

Further, FIG. 5 illustrates a read line 568, via which in embodiments ofthe present invention pixel or detection means 420 arranged, forexample, in rows and columns can be connected to the downstreamevaluation means 470 by respective control by a control logic.

The evaluation means 470 can have, for example, an operational amplifier280, wherein an “inverting input” 280 b of the operational amplifier 280can be connected to the output 280 a of the operational amplifier via aswitch 282 that can be controlled with a signal Φ₃. The non-invertinginput 280 c of the operational amplifier 280 can be on a referencepotential 275. First 274 a and second 274 b sample capacitances of theevaluation means 470 connected in parallel are connected to the input ofthe evaluation means 470 via first 572 a and second 572 b sampleswitches. The first 572 a and second 572 b sample switches can each beopened or closed in response to a signal Φ_(1a) or Φ_(1b). The twosample capacitances 274 a and 274 b are connected to a node 575 at theirother terminal side, which provides, on the one hand, an electricalconnection to the “inverting” input 280 b of the operational amplifier280, and, on the other hand, an electrical connection to two feedbackcapacitances C_(F1) 284 a and C_(F2) 284 b connected in parallel.Additionally, the node 575 can be coupled to the output 280 a of theoperational amplifier via the switch 282. The two feedback capacitances284 a and 284 b are connected to the input 280 a of the operationalamplifier 280 via the amplifier switches 288 a and 288 b. The twofeedback capacitances 284 a and 284 b connected in parallel areadditionally electrically coupled to a reference potential 275 via areference voltage switch 290 a and 290 b. The reference voltage switches290 a and 290 b can be closed or opened in response to a signal Φ₅. Thetwo amplifier switches 288 a and 288 b can be opened or closed inresponse to a signal Φ_(2a) or Φ_(2b). The output 280 a of theoperational amplifier is connected to the output 299 of the evaluationmeans, wherein the output 299 of the evaluation means 299 cansimultaneously correspond to an output of the inventive optical distancemeasuring device or the inventive apparatus. At this output, theevaluation result or part of the evaluation result can be output in theform of a voltage U_(out) or be passed on for further processing. Thiscan, for example, be a differential voltage signal.

In a further embodiment of the present invention, the apparatus or theoptical distance measurement device, as is indicated in FIG. 5, cancomprise a control logic 580, which can be implemented such that theabove-mentioned shutters and switches 227, 252 a, 252 b, 560 a, 560 b,255, 572 a, 572 b, 282, 288 a, 288 b, 290 a, 290 b are controlled suchthat operation of the detection means and the evaluation means, asdescribed in embodiments of this invention, is made possible.

The mode of operation of the above embodiment of FIG. 5 is illustratedin the schematic time circuit diagram in FIG. 6. In the circuit diagramsa-n in FIG. 6, the time t is plotted on the x axis in arbitrary units,and on the y axes of the diagrams a-n signal impulses, light impulses,voltages or switching states in arbitrary units. In this description ofthe mode of operation of the circuit of FIG. 5 it is assumed thatdetection means 420 or inventive apparatuses can be arranged in a matrixin rows and columns, wherein one evaluation means 470 can be associatedwith each column and the detection means 420 can be read out orevaluated row by row by these evaluation means 470 associated column bycolumn. One embodiment regarding an overall arrangement will be givenbelow in connection with FIG. 7.

In response to a reset signal 334, a defined reset voltage V_(reset) 229is applied to all pixels 125 a, so that their photodiodes are set to adefined voltage. After all pixels have been reset to a predefined state,the shutter 252 a and 252 b (FIG. 5) is closed with temporally knowncoupling, e.g. simultaneously, as illustrated in this embodiment withthe emission of a laser pulse 304 with a pulse duration 304 a (diagramb). Closing the shutter 252 a, i.e. electrically connecting the pixelelement with the sample and hold circuit 550 for the first shutter 252 a(FIG. 5) can be performed for a first period 304 a corresponding to thepulse width t_(p) of the laser pulse 304. The integration time t_(int1)for this first time window 314 or the first detection period can hencecorrespond to the pulse duration t_(p) of the emitted laser pulse 304.Generally, however, t_(int1)≧t_(p) applies. The second time window 316of the second shutter 252 b closed simultaneously with the first shutter252 a has a duration t_(int2) 316 a in this embodiment which is longerthan the first detection period 314 with the duration t_(int1) 304 a.

As is illustrated in diagrams f and g in FIG. 6, after opening the timewindows 314 or 316 and hence after electrically connecting the pixelelement 125 a to the sample capacitances 254 a or 254 b of the sampleand hold circuit 550, the voltages U₁ 318 or U₂ 322 are formed at thetwo sample capacitances. Since the first detection period 314 cancorrespond to the duration of the laser pulse and is hence shorter thanthe second detection period 316, the sample capacitance 254 a, afterclosing the time window 314 or opening the shutter 252 a, has a voltageU₁ that is different from the voltage U₂ app lied to the second samplecapacitance 254 b connected in parallel after closing the second timewindow 316. Since the laser pulse 310 reflected at an object impinges onthe pixel element 125 a with the pulse duration 304 a delayed by theruntime 303, the voltage U₁ 318 applied to the sample capacitance 254 aafter closing the first time window 304 contains information on thedistance to the object. Additionally, the voltage U₁ 318 includesextraneous light or background light portions, as well as information onreflectance r and sensitivity R of the photodiode. For the voltage U₁,the connection shown in equation (1) applies. Since the second timewindow 316 has a duration t_(int2) that is higher than the pulse widtht_(p), i.e., for example, higher than the pulse duration t_(p) and theruntime 303 t_(run), the second voltage U₂ includes information on thewhole laser pulse 310 reflected on the object. Since the whole reflectedlaser pulse was integrated or detected, the voltage U₂ includes no moreinformation on the distance to the object. The second voltage U₂ appliedto the second sample capacitance 254 b includes information on thereflectance r or the sensitivity R of the pixel. Additionally, itincludes information on the background signal or an extraneous lightportion 324.

In other words, by implementing the sample and hold circuit 550 and arespective control, of the overlapping different time windows 314 and316 in parallel, the two sub-cycles I_(A) 306 and II_(A) 307 of FIG. 3can be captured in parallel, i.e. with a single laser pulse.

In diagrams h-n in FIG. 6, the mode of operation of the differentswitches of the circuit illustrated in FIG. 5 during distancemeasurement will be discussed. Prior to the whole accumulation cycle,switches 290 a and 290 b are closed once with the help of the controlsignal Φ₅ in order to charge the feedback capacitances 284 a and 284 bto a reference voltage 275. The switches 290 a and 290 b are openedagain prior to the synchronous reset 334 for all pixels of the detectionmeans 420. After detecting the reflected laser pulse in the twooverlapping detection periods 314 and 316, the select switch 255 (FIG.5) is closed (diagram h), which establishes a connection of thedetection means 420 with the evaluation means 470. In a phase A, thecontrol signal Φ_(1a) is logically set to “1” together with the selectsignal, so that the switch 560 a is closed. The switch 572 a in theevaluation means 470 is also closed via the control signal Φ_(1a), sothat a conductive connection is formed between the first samplecapacitance 254 a in the sample and hold circuit 550 and the firstsample capacitance 274 a in the evaluation means 470. In the phase A,the switch 282 is closed by the control signal Φ₃. Thereby, on the onehand, the operational amplifier 280 is short-circuited and, on the otherhand, the feedback capacitances 284 a and 284 b are charged to an offsetvoltage of the operational amplifier. In this phase, the samplecapacitor 274 a charges to a differential voltage between the inputvoltage U₁ 318 and the reference voltage 275 of the operationalamplifier 280, minus an offset voltage of the operational amplifier.

Analogously, during a second phase B, by opening switches 560 a and 572a and by closing switches 560 b and 572 b in response to a controlsignal Φ₃ and Φ_(1b), the second sample capacitance 274 b is set to adifferential voltage given by the input voltage U₂ 322 minus thereference voltage 275 minus the offset voltage of the operationalamplifier 280. In a third phase C, the switch 282 is opened again inresponse to the control signal Φ₃, i.e. a conductive connection isinterrupted. In this phase, switches 560 a and 572 a are also closed bythe control signal Φ_(1a), and the feedback capacitance 284 is coupledto the operational amplifier 280 by closing the switch 288 a in responseto a control signal Φ_(2a).

In this phase, a first row of pixels is set to a defined starting stateby a reset signal 338, in order to detect the background signals in thesubsequent dark period. With applying the reset pulse 338, the radiationpulse period is terminated simultaneously, which begins in thisembodiment with the start of the integration in the integration windows314 and 316 and in which both the distance and the reflectance and thebackground signal can be captured with a single laser pulse 304. In thisembodiment, the start of the laser pulse coincides with the start of theintegration, generally, however, the start time of the laser pulse canbe different to the start time of the integration. In the subsequentdark period, a simultaneous closing of the shutter 252 a for a firstdetection period 315 of the dark period having a duration t_(int3) 304 aand closing of the second shutter 252 b for a second detection period ofthe dark period 317 with a duration t_(int4) 316 a is performed. In thisembodiment, the duration t_(int3) corresponds again to the pulseduration t_(p) of the laser pulse in the radiation pulse period and theduration t_(int4) of the time window 317 corresponds to the duration 316a of the time window 316 for integrating the whole reflected laserpulse. Generally, however, the duration of the integration window 314,315 can be different to the pulse duration t_(p). Then, analog to theabove description, a voltage is formed at the sample capacitances 254 aand 254 b, which, however, in the dark period, corresponds only to thebackground light portion together with the reflectance r and thesensitivity R of the photodiode in the respective integration periods.In other words, during this dark period, the sub-cycles I_(B) 308 andII_(B) 310 mentioned in connection with FIG. 3 are performed inparallel. In this context, the contiguous radiation pulse period, i.e.the period where the integration for detecting the reflected radiationpulse is performed, can stretch to the beginning of the contiguous darkperiod, which follows in this embodiment. The dark period can begin withopening at least one of the two overlapping time windows, during theperiod where no radiation pulse is transmitted, and can be terminatedafter detecting the background light portion together with thereflectance r and the sensitivity R of the sensor. In other words, theradiation pulse period can begin with opening at least one of the twooverlapping time windows for detecting the reflected radiation pulse andcan be terminated after closing the last of the two overlapping timewindows for detecting the whole reflected laser pulse.

During the phase C, a voltage U_(1,extraneous) 320, which corresponds tothe background signal during the time window 315, is formed at thesample capacitor 274 a of the evaluation means 470. In this phase, theoffset voltage, which is again applied to the feedback capacitor 284 a,compensates the compensation voltage or offset voltage of theoperational amplifier 280 and a differential voltage between U₁ andU_(1,extraneous) plus a possible reference voltage 275 is applied to theoutput 299 of the evaluation means 470.

During the subsequent phase D, analogously, a voltage signal is outputat the output 299 of the evaluation means 470, which corresponds to thedifference of voltages U₂ and U_(2,extraneous), plus a reference voltage275. If the input 280 c of the operational amplifier 280 is connected toground, the reference voltage 275 corresponds to ground, and thus therelations derived in equation 3 or equation 6 result for thedifferential voltages applied to the output 299 in phases C and D. Thesedifferential voltage values can then be further processed, for example,in a camera system or on the chip or sensor, i.e. “on-chip”, such thatthe quotient of the two differential voltages U_(1,diff) and U_(2,diff)is formed, and therefrom, as illustrated in equation 7 and equation 8,the distance of a pixel to a corresponding object point can becalculated.

Therewith, the parallel evaluation of all pixel elements arranged in afirst row is completed in the evaluation means 470 associated parallelthereto column by column, and, by changing the select signal (diagram h)at the end of phase D, a second row in the pixel array can be read intothe associated evaluation means 470. Since the initial reset signal 334has been performed for all pixels of the array, the respective voltagevalues U_(1,row2) 630 and U_(2,row2) 632 for distance, reflectance,sensitivity and background portions during the radiation pulse are stillon the sample capacitances 254 a and 254 b of the detection means 420.These voltage signals of the pixel are further processed in the newlyselected 2″ row in phases E and F analogously to the phases A and B inthe evaluation means 470. By a row reset signal 610 for the CDS stage,voltages U_(1,extraneous,row2) 635 and U_(2,extraneous,row2) 637regarding the background light portion, the reflectance r and thesensitivity R are detected in respective time windows 612 and 614 inphases G and H and further processed as described above. Successively,the residual rows of the pixel array can be read out analogously.

In the embodiment of the present invention illustrated in FIG. 5, thesample and hold circuit 550 for simultaneously capturing the distanceand reflectance components is implemented twice. Likewise, the samplecapacitances 274 a and 274 b and the feedback capacitances 284 a and 284b of the evaluation means 470 exist twice. In this embodiment, theintermediate amplifiers or buffer amplifiers 134 and 264 each exist onlyonce. In another embodiment, these intermediate amplifiers can also beimplemented twice or in parallel, whereas the sample capacitances 274 aand 274 b can also be combined to one sample capacitance 274.

Contrary to the conventional apparatus and the method illustrated inconnection with FIGS. 2 and 3, where the shutter signals for closing andopening the time windows were applied one after the other, with theembodiment illustrated in FIGS. 5 and 6 shutter signals for the timewindows for integrating a signal formed in dependence on the receivedlight can be applied in parallel. The voltage currently applied acrossthe photodiode 130 is detected four times in the overall measurementcycle. First, by means of shutter 1 252 a, the signal proportional tothe distance for the signal cutting the laser pulse is stored on C_(H)1254 a. Then, after a maximum waiting time that depends on themeasurement range depth, with the help of shutter 2 252 b the signalproportional to the reflectance and sensitivity corresponding to thecomplete reflected laser pulse is stored on C_(H2) 254 b. For obtainingthe different signal portions and the correction of the additivebackground light and the components of the multiplicative error by theobject reflectances, equations 1-8 apply. Obtaining the distance andreflectance signals is performed in a temporally parallel manner and nolonger one after the other. The voltages U₁ 318 and U₂ 322 each includean additive extraneous light portion E_(extraneous) and a multiplicativeportion depending on the reflectance r of the detected object point (seeequation 1 and equation 2). These analog signals are subtracted fromeach other directly on the image sensor chip, in the evaluation means470, with very high accuracy. This is performed in the evaluation means470, which is a circuit for correlated double sampling (CDS stage). Themeasurement cycle illustrated in FIG. 6 includes the picture of acomplete three-dimensional depth chart. It begins with simultaneouslyexposing all pixels in a distance measurement phase. As representativefor all pixel signals, the voltages U_(1,2) are shown for oneimplemented pixel of the sensor matrix. After switching off or openingthe shutter 1 252 a and the shutter 2 252 b, the voltages proportionalto the distance or reflectance corresponding to the respectivelydetected signals are applied to the capacitances C_(H1,H2) (254 a and254 b). Then, the pixel matrix is read out row by row to the associatedcoupling or memory capacitance C_(F1,F2) in the CDS stage in the phasereadout row 1, readout row 2, . . . up to readout row K with K sensorrows. In this embodiment (FIG. 6) this applies in a column-parallelarrangement of the CDS stages, as here all values of a row K aretransmitted simultaneously to the respectively associated columnamplifiers. The clock signals Φ_(1a,1b) and Φ₃ transmit the signalsproportional to the laser to the column or sample capacitances C_(S1,S2)274 a, 274 b in phases A and B. Then, in phases C and D, the pixels ofthe respectively selected row are reset by means of reset 338, in orderto obtain the additive background light portions both of the signalproportional to the distance and the signal proportional to thereflectance. In phases C and D, these signals are stored on samplecapacitances C_(S1,D2) 274 a, 274 b. Simultaneously, by activating theclock signals Φ_(2a,2b), the difference of the voltages from phases Aand C and B and D is stored on the hold or feedback capacitancesC_(F1,F2) 284 a, 284 b. For this purpose, C_(F1,F2) were previouslyreset by means of the clock signal Φ₅. During a multiple accumulation,resetting can be performed once at the beginning of the accumulationsequence in or prior to the radiation pulse period in the distancemeasurement phase 620. In the subsequent accumulation sequence, no resetis performed any more. In FIG. 6, the case for a single accumulation isshown, but the presented double shutter method is also suitable formultiple accumulation without limitation. Since the measurement in theinventive methods or in the inventive apparatus is performedsimultaneously, also for the reflectance values, the extraneous lightportions due to the background light are correlated with almost 100%,and the portions due to the reflectance with almost 100%. These voltagesproportional to the reflectance r can be read out serially in thefurther process and the quotient of the voltages U_(1,diff) andU_(2,diff) can be calculated according to equation 7 externally in thecamera system or on-chip.

FIG. 7 shows a simulated time circuit diagram, which is to be discussed,based on the embodiment of FIG. 5. In the circuit diagrams a-k, in FIG.7, a time t is plotted in microseconds on the x-axis. In thissimulation, the time axes are, for example, plotted from 1.24 μs to 8.64μs. The respective y-axes of the diagrams a-k represent light impulses,signal impulses, voltage or switching states in arbitrary units.

Prior to the begin of the accumulation cycle, as illustrated in diagramj, switches 190 a and 290 b (FIG. 5) are closed in order to charge thefeedback capacitance 284 a and 284 b to a defined reference voltage 275.In a different embodiment, the circuit can only have one switch 290 forcharging the feedback capacitances to a defined reference voltage 275 inresponse to a control signal (N. Prior to emitting the laser pulse(diagram a), a synchronous pixel reset takes place for all pixels. Thisis indicated in FIG. 7 in diagram b by the pixel reset value set to alogical “1”. As has already been described above, the barrier layercapacitance 132 (FIG. 5) connected in parallel to the photodiode is setto a defined reset voltage value 229 by the pixel reset. In diagrams dand, the shutter 1 252 a and the shutter 2 252 b are closed partlytemporally overlapping with the pixel reset signal. I.e. they are on alogical “1” level. From time A onwards, after the pixel reset isterminated and thus goes to a logical value “0”, i.e. the respectivereset switch 227 is opened, the voltage value at the barrier layercapacitance 132 decreases, and likewise, since shutters 1 and 2 areclosed, at the respective sample capacitances C_(H1) 254 a and C_(H2)254 b. Starting from the time A, the integration phase or opening of thefirst and second time windows for integrating the laser pulse 304begins. At a time B, the first integration period lasting from time A toa time B having a first duration t_(int1) is closed and a respectivevoltage U₁ is temporarily stored on a sample capacitance, for example254 a. The temporarily stored voltage U₁ includes, as already mentionedabove, information on the distance, reflectance and sensitivity of thesensor and the background light portion. The first duration t_(int1)given by the period between times A and B is to be selected such thatthe integrated reflected laser pulse is partly cut off to obtaindistance information on the object to be measured. The second switch 252b (FIG. 5) can then be closed at a time C, as is shown in diagram e.Thereby, the duration of the second integration time t_(int2) is longerthan the first duration t_(int1) of the first detection period and longenough to integrate the whole reflected laser pulse. I.e. theintegration time t_(int2) given by the period between the times A and Cis longer than a pulse width of the emitted laser pulse. After closingthe second window, i.e. opening the second shutter 252 b, a respectivevoltage U₂ is “frozen”, for example on the sample capacitance C_(H2) 254b. Since the shutter window has a duration t_(int2) that is longer thanthe duration of the laser pulse and hence the whole pulse form of thelaser is integrated, this voltage value U₂ contains no longer anydistance information since this is obtained by “cutting off” thereceived laser pulse. Rather, again the background light portion and thelaser portion are captured together with information on reflectance andsensitivity of the sensor. After closing the second time window oropening the second shutter, the radiation pulse period is terminated inthis embodiment. The respective voltage values U₁ and U₂ are temporarilystored on the sample capacitances 254 a and 254 b.

As is illustrated in diagram f (FIG. 7), by closing the switch 560 a,which, in deviation from the embodiment in FIG. 5, has no AND logicgate, a connection with a CDS stage can be made. Thus, in embodiments,it is not necessitated that the circuit comprises a select switch 255.Simultaneously with the switch 560 a, the switch 572 a is closed, sothat the voltage U₁ can be formed at the sample capacitance 274 a of theevaluation means 470. As is illustrated in diagram h, the switch 282 isclosed by the control signal Φ₃ prior to and during integration, inorder to charge the voltage value on the feedback capacitances 284 a and284 b to an offset voltage of the operational amplifier 280 and tostabilize the same. Opening of the switch 282 at the time D and renewedclosing of the switch 282 at the time D′ shown in diagram h can also beomitted. At the time E, the switch 282 is opened in response to acontrol signal Φ₃, the logical value is therefore “0”. In this phase, apixel reset takes place again (diagram b) and the shutter 1 252 a isalso closed again (diagram d), so that the reset voltage V_(reset) 229can again be formed at the barrier layer capacitance 132 of the pixel125 and the sample capacitance 254 a. By closing the switch 288 inresponse to a control signal Φ₂, as is illustrated in diagram i, thefeedback capacitance 284 a is again coupled to the operational amplifier280. Thereby, the feedback capacitor 284 a charges to a differentialvoltage, which is given by the voltage at the sample capacitor 274 a andthe reference voltage 275 of the operational amplifier 280 minus astarting voltage of the operational amplifier. This voltage is appliedto the output of the CDS stage 299, as can be seen in diagram k. Afterterminating the pixel reset at time F and opening the shutter 1 252 a attime G, the integration of the first time window is terminated without alaser pulse, i.e. in the dark period. Since the switches 560 a and 572 aare still closed, as can be seen in diagram f, the voltageU_(1,extraneous), i.e. the voltage without laser pulse portion, isapplied to the sample capacitor 274 a of the evaluation means 470, suchthat the difference of voltages U_(1,diff)=U₁−U_(1,extraneous) is formedat the time G at the output of the CDS stage, as can be seen in diagramk.

In the following, the CDS stage is reset by opening the switch 288 by acontrol signal Φ₂ and by closing the switch 282 in response to a controlsignal Φ₃. Beforehand, by opening switches 560 a and 572 a, the samplecapacitance 274 a of the CDC stage was decoupled from the samplecapacitance 254 a of the sample and hold circuit. By closing switches560 b and 572 a, analogously to the voltage value U₁ also the voltage U₂that is still temporarily stored in the sample capacitance 254 b is nowtransmitted to the CDS stage. By closing the switch 227, a pixel resetis performed again in the period H to I and analogously the secondshutter 252 b is closed, so that the background light portion isdetected without a laser pulse in the second detection period t_(int2),here ranging from time I-J. The respective voltage signal is thentransmitted to the CDS stage while switches 560 b and 572 a are stillclosed, and there, analogously to the voltage difference U_(1,diff), thedifferential voltage U_(2,diff) is formed which is then applied to theoutput 299 of the CDS stage from the time J onwards. The formation ofthis voltage value U_(2,diff) is no longer shown in the simulation.

In this embodiment, the subtraction of the respective voltage valuestakes place serially of the CDS stage. This means that first thedifferential voltage value U_(1,diff) is formed with and without laserpulse portion for the time window having a shorter integration timet_(int1), and subsequently, serially, the differential voltageU_(2,diff) is formed for the second time window with the longerintegration period t_(int2), with and without laser pulse. For thisreason, the CDS stage in this embodiment can be implemented seriallyinstead of in parallel, i.e. the same can have only one switch 572 andone sample capacitance 274 as well as one switch Φ₂ and Φ₅ and only onefeedback capacitance 284.

As a further embodiment, FIG. 8 shows the circuit diagram of a detectionmeans 420 and an associated evaluation means 470. In this embodiment,the evaluation means 420 consists again of a pixel element 125 a, whichis structured, as already described in the context of FIG. 5, i.e., aphotodiode 130 having a barrier layer capacitance 132 connected inparallel is connected to ground 133 with one terminal and connected to abuffer amplifier 134 and the pixel reset switch 227 with the otherterminal. The pixel reset switch 227, also called reset switch 227 inthis application, charges the pixel capacitance in the closed state tothe predefined reset voltage V_(reset) 229. The buffer 134 can be avoltage follower having an amplification of x1 or also a buffer possiblyhaving a higher amplification than x1. Apart from that, the detectionmeans 420 has a sample and hold circuit 550, which is connected to theoutput of the buffer 134 and hence to the pixel element 125 a via theswitch 252 and the shutter. In this embodiment, the sample and holdcircuit 550 has two sample capacitance 254 a and 254 b that are eachconnected to ground 133 with one terminal and connected to the input ofthe sample and hold circuit with the other terminal each via switches 80a and 80 b via the shutter 252. Hence, in this embodiment, the samplecapacitances 254 a and 254 b are not arranged in parallel, but arecoupled to a common signal path via a node, as can be seen in thecircuit diagram in FIG. 8. Additionally, the two sample capacitances 254a and 254 b are again connected to a subsequent circuit for correlateddouble sampling (CDS stage) 470 via a select switch 255.

The circuit for (correlated) double sampling 470 has a hold capacitance276, which is, on the one hand, connected to ground 133 and, on theother hand, connected to the buffer amplifier 264 at the input of theCDS stage 470. The circuit for (correlated) double sampling 470 has asample capacitance C_(c10) 274 b, which is, on the one hand, connectedto the hold capacitance 276 via a switch 572 b via the node 85 and, onthe other hand, is connected to the output of the buffer amplifier 264.Optionally, a second parallel branch with a switch 572 a and a secondsample capacitance 274 a can be arranged. However, this parallel branch84 is optional, since the evaluation in the CDS stage 470 is performedserially if only one operational amplifier 280 per double shutter pixel420 is used. If the CDS stage has only one sample capacitance, thedetection of the background light portion in the dark period can only beperformed serially, i.e., the respective detection periods do notoverlap as shown in diagrams f and g in FIG. 7. In two overlappingdetection periods during the dark period, two sample capacitances 274 aand 274 b are necessitated for temporarily storing the voltage values inthe CDS stage, as is shown in FIG. 5 and FIG. 8 (optional). Further, theevaluation means 470 has again two feedback capacitances 284 a and 284 barranged in parallel and connected to the sample capacitances 274 a(optional) and 274 b via a node 575. Further, the node 575 is connectedto the “non-inverting” input 280 b of the operational amplifier 280. Viathe switch 282, the operational amplifier can be short-circuited inresponse to a control signal Φ₃. The feedback capacitances 284 a and 284b can be connected to the output 280 a of the operational amplifier viathe feedback switches 288 a and 288 b, or can be connected to thereference potential 275 via a reference potential switch 290, inresponse to a control signal Φ₅. The output voltage V_(out) of theevaluation means 470 is applied to the output 299, wherein the outputsignal can be smoothed via an output capacitor C_(out) 89. The invertinginput 280 c of the operational amplifier 280 can be on an offsetpotential 276.

This embodiment is distinguished by a space requirement reduced byapprox. 50% and a power consumption reduced by approx. 50% per doubleshutter pixel compared to conventional pixels with a sample and holdcircuit. In this embodiment, after closing the shutter 252 in the samplecapacitances 254 a and 254 b, the reflections of the radiation pulse atan object surface and a background radiation are captured together withthe reflectance and sensitivity portions during the radiation pulseperiod once in a short integration time t_(int1), i.e. in a firstshutter time, and in a second integration time t_(int2) that is longerthan the first integration time and overlaps with the same, theinfluence of the whole reflected radiation pulse on the voltage iscaptured, i.e., a short and a long shutter time are realized in onepixel path that is, contrary to the embodiment in FIG. 5, notimplemented in parallel. One advantage of this embodiment is, as hasalready been mentioned above, the reduced space and power requirement,as well as an improved noise behavior, since the reset noise and the SF1noise is the same for both shutter times (integration time t_(int1),t_(int2)). The voltage values U₁ and U₂ applied to the samplecapacitances 254 a and 254 b can be evaluated at the CDS stage 470, asdescribed above. In this embodiment, the whole distance measurement witha short and a long shutter time, each with and without a laser pulse,can be detected in one pixel path. Additionally, the background lightfor all pixels of a pixel array can be detected within one shutter timeand does not have to be performed serially row by row as in otherembodiments, which can lead to a shortening of the measuring time forthe distance measurement. Since, as will be discussed in more detailbelow, the laser and background light portions are detected within oneshutter time, the correlation is increased and can therefore be almost100%. Additionally, the measurement time is shortened compared to otherembodiments and conventional methods.

In a further embodiment, FIG. 9 shows a detection means 420 in the formof a circuit diagram. Again, the detection means 420 comprises a pixelelement 125 a as well as a sample and hold circuit 550. In thisembodiment, the sample and hold circuit 550 has four sample capacitancesC_(S0), C_(S1), C_(S2) and C_(S3) with the respective numbering 254a-254 d. The same are connected to ground on one terminal side and onthe other terminal side connected to the shutter 252 via nodes of thepixel path each via select switches 80 a-80 d.

The same is coupled to the pixel element 125 a via the buffer amplifier134. Further, the sample and hold circuit 550 has again a select switch255 that can couple the sample capacitances to a downstream evaluationmeans 470 via their respective select switches 80 a-80 d. Again, theevaluation means 470 can be a CDS stage, as described in embodiments,for example in FIG. 8. With this circuitry for the sample circuit it ispossible to reduce the space and power requirement compared toconventional sample and hold circuits for distance measurement and,additionally, both the reset noise and the noise SF1 for all sampledvoltages is the same, both for the background and for the laserexposure. As will be shown below in FIGS. 10 and 11, with thisembodiment, the background light in a dark period can also be detectedfor all pixels within one shutter time and does not have to be read outserially row by row. This can reduce the measurement time and thecorrelation of the laser and background light within one shutter timecan be increased to almost 100%. The whole measurement with a short anda long shutter time, with and without a laser pulse, can be realized inone pixel path and within one shutter time. Additionally, the backgroundlight portion can be subtracted with an extrapolation method, as will bedescribed below in more detail.

FIG. 10 illustrates in a diagram how, according to a further embodimentof the present invention, the background light can be determined with anextrapolation method. In FIG. 10, the time t is plotted in arbitraryunits on the x axis, and the voltage signal U at the barrier layercapacitance 132 of the photodiode 130 is plotted on the y axis. Afterterminating a pixel reset at the time t′, a predefined voltage valueV_(reset) is applied to the barrier layer capacitance of the pixelelement 125 a. In the dark period following in this embodiment, where nolaser pulse is emitted, the voltage decreases merely by the backgroundlight. For extrapolating the background light, the time curve of thisvoltage is measured at four measurement points U₀, U₁, U₂ and U₃ andtemporarily stored on respective sample capacitances C_(s0), 254 a,C_(s1) 254 b, C_(s2) 254 c and C_(s3) 254 d (FIG. 9). As is furthershown in FIG. 10, first, at a time t₀, the voltage U₀ is temporarilystored on a first sample capacitance, for example 254 a. The storage ofthe individual values takes place, as has already been described, byrespectively closing the switches 80 a-80 d associated to the samplecapacitances. At a second time t₁, the voltage value U₂ is temporarilystored on a second sample capacitance 254 b. Between the times t₀ andt₁, a time interval of a length τ₀ has passed. Then, the individualvoltage values are temporarily stored with the help of a sensor controlor controller by controlling and opening the respective select switches80 a-80 d at the respective time, and “freezing” the respective voltageson the respective sample capacitances.

After temporarily storing the voltage value U₁, the dark period isterminated. For the subsequent radiation pulse period, in which thereflected laser pulse 304 is detected, a short integration periodτ_(short) is started by closing, for example, switch 80 c, and a longintegration period τ_(long) by closing switch 80 d in a phase-lockedmanner with respect to the laser pulse 304. The shorter of the twodetection periods in this radiation pulse period is closed after anintegration time τ_(short), such that the reflected laser pulse ispartly “cut off” in order to obtain the desired distance information.For this purpose, the voltage U₂ is further temporarily stored, forexample on the sample capacitance C_(s2), when closing the switch 80 c.The second time window with an integration time τ_(long) is longer thanthe duration of the laser pulse, so that the whole pulse form of thelaser is integrated. The respective voltage value U₃ is then temporarilystored, for example, on the sample capacitance 254 d. The voltage valuesU₀, U₁, U₂ and U₃ can be measured as described above, and the values ΔU₂and ΔU₃, i.e. the “net signals” due to the laser exposure that areneeded for calculating the distance of an object can be calculatedtherefrom. The background light portion of the voltage decrease can beextrapolated by the following equation:

$\begin{matrix}{{U_{background}(t)} = {U_{0} - {\frac{U_{0} - U_{1}}{\tau_{0}} \cdot t} + {\frac{U_{0} - U_{1}}{\tau_{0}} \cdot t_{0}}}} & (9)\end{matrix}$

In equation 9, the voltage values U₀ and U₁ can be measured and the timevalues t₀ and τ₀ can be adjusted or are known. With the help of equation9, ΔU₂ can be calculated according to the following equation:

$\begin{matrix}{{\Delta \; U_{2}} = {U_{2} - U_{1} + {\left( {U_{0} - U_{1}} \right) \cdot \frac{\tau_{short}}{\tau_{o}}}}} & (10)\end{matrix}$

In the same manner, ΔU₃ can be calculated:

$\begin{matrix}{{\Delta \; U_{3}} = {U_{3} - U_{1} + {\left( {U_{0} - U_{1}} \right) \cdot \frac{\tau_{long}}{\tau_{o}}}}} & (11)\end{matrix}$

The double shutter principle with a single exposure time can also beapplied to the background light compensation. All pixels of a pixelarray can compensate the background light portion with the help of thisextrapolation method in a single dark period. As has been describedabove, the voltage values temporarily stored on the sample capacitances254 a-254 d can be subtracted in the following CDS stage 470 in order toobtain the distance information.

FIG. 11 a illustrates in a further embodiment a more general version ofthe extrapolation method for the background light portion. In thisembodiment, the second voltage U₂ (FIG. 11 a) for the extrapolatedbackground light is not sampled at the starting point of the integrationof the laser pulse during the radiation pulse period. This can beperformed at any time within the previous dark period.

As described in the context of FIG. 10, the time t is plotted inarbitrary units on the x axis, and the voltage applied to the photodiodein arbitrary units on the y axis. Contrary to the embodiment in FIG. 10,the voltages U₁ and U₂ can be measured at any time in the dark periodand temporarily stored on the respective sample capacitances of thesample and hold circuit 550. For this purpose, for example at time t₁,the voltage U₁ can be temporarily stored on the sample capacitor C_(s0)(FIG. 9), and a second voltage value U₂ on a second sample capacitance,for example C_(s1), at a second time t₂ in the dark period. It followsthat in this embodiment the voltage changes at the photodiode due to thebackground light portion are measured in the dark period in twooverlapping detection periods and “frozen” based on two voltage valuesU₁ and U₂. In a radiation pulse period, then two detection periods forthe laser pulse are coupled in a phase-locked manner at the time ofemitting the laser 304. A first detection period having an integrationtime τ_(short) or t_(int) partly cuts off the reflected laser pulse,i.e. the time window is closed before the laser pulse has beencompletely integrated. Rather, this takes place in the second detectionperiod having a duration τ_(long) or t_(int2) that is at least as longas the laser pulse. After closing the first detection period forintegrating the reflected laser pulse at the time t₃, the respectivevoltage U₃ is temporarily stored, for example, on the sample capacitanceC_(s2). This means that the voltage value U₃ including the distanceinformation to the object is applied to this sample capacitance. Thevoltage value U₄ is then temporarily stored, for example, on the samplecapacitor C_(s3) by closing the second time window having the durationτ_(long). The starting times of the first and second time windows forthe radiation pulse period can be phase-locked with the laser pulse 304.

By this method, the voltages U₁ to U₄ can be temporarily stored on therespective sample capacitances. Then, the net signals can be determinedbased on the laser exposure, i.e. ΔU₃ and ΔU₄.

For changing the voltage by the background light portion, the followingfunctional context applies:

U _(background)(t)=mx+b=:U _(H)(t)  (12)

The functional context between voltage and time can, therefore, bedescribed by a linear equation. Applied to FIG. 11 a, this means thatthe slope of the straight line can be illustrated by the voltagedifference of the voltage values U₂ and U₁ divided by the respectivetime differences t₂−t₁ at a respective time t minus time offset t₁ andvoltage U₁.

This is illustrated in equation 13:

$\begin{matrix}{{{U_{H}(t)} - U_{1}} = {\frac{\left( {U_{2} - U_{1}} \right)}{t_{2} - t_{1}} \cdot \left( {t - t_{1}} \right)}} & (13)\end{matrix}$

This equation can be rearranged as follows:

$\begin{matrix}\begin{matrix}{{U_{H}(t)} = {{{\frac{U_{2} - U_{1}}{t_{2} - t_{1}} \cdot t} - {\frac{U_{2} - U_{1}}{t_{2} - t_{1}} \cdot t_{1}} + U_{1}} =}} \\{= {{{\frac{U_{1} - U_{2}}{t_{2} - t_{1}} \cdot t} + {\frac{U_{1} - U_{2}}{t_{2} - t_{1}} \cdot t_{1}} + \frac{{U_{1}t_{2}} - {U_{1}t_{1}}}{t_{2} - t_{1}}} =}} \\{= {{{\frac{U_{1} - U_{2}}{t_{2} - t_{1}} \cdot t} + \frac{{U_{1}t_{1}} - {U_{2}t_{1}} + {U_{1}t_{2}} - {U_{1}t_{1}}}{t_{2} - t_{1}}} =}} \\{= {{{- \frac{U_{1} - U_{2}}{t_{2} - t_{1}}} \cdot t} + \frac{{U_{1}t_{2}} - {U_{2}t_{1}}}{t_{2} - t_{1}}}}\end{matrix} & (14)\end{matrix}$

so that the following linear equation results for the functional contextof voltage U_(H)(t):

$\begin{matrix}{{U_{H}(t)} = {{{- \frac{U_{1} - U_{2}}{t_{2} - t_{1}}} \cdot t} + \frac{{U_{1}t_{2}} - {U_{2}t_{1}}}{t_{2} - t_{1}}}} & (15)\end{matrix}$

With this functional context, the voltage value can be calculated orextrapolated based on the pure background light portion at any time t.

ΔU₃ can then be easily determined by using the temporarily storedvoltage value U₃ and equation 15. For ΔU₃, the following results:

$\begin{matrix}{{\Delta \; U_{3}} = {{U_{3} - {U_{H}\left( t_{3} \right)}} = {U_{3} + {\frac{U_{1} - U_{2}}{t_{2} - t_{1}} \cdot t_{3}} - {\frac{U_{1} - U_{2}}{t_{2} - t_{1}} \cdot t_{1}} - U_{1}}}} & (16)\end{matrix}$

After further rearrangement, equation 17 results:

$\begin{matrix}\begin{matrix}{{\Delta \; U_{3}} = {{U_{3} - U_{1} + {\frac{U_{1} - U_{2}}{t_{2} - t_{1}} \cdot \left( {t_{3} - t_{1}} \right)}} =}} \\{= {U_{3} - U_{1} + {\left( {U_{1} - U_{2}} \right) \cdot \frac{t_{3} - t_{1}}{t_{2} - t_{1}}}}}\end{matrix} & (17)\end{matrix}$

ΔU₃ can therefore be determined by the difference of voltage values U₃and U₁ and the difference of voltage values U₁ and U₂, as well as therespective differences of the times t₃−t₁ divided by t₂−t₁. These timescan be adjusted by the sensor control and can be considered to be known,since the same correspond to the respective time window of the shutter.Analogously, ΔU₄ can be calculated, such that equation 18 results:

$\begin{matrix}{{\Delta \; U_{4}} = {{U_{4} - {U_{H}\left( t_{4} \right)}} = {U_{4} - U_{1} + {\left( {U_{1} - U_{2}} \right) \cdot \frac{t_{4} - t_{1}}{t_{2} - t_{1}}}}}} & (18)\end{matrix}$

ΔU₄ can again be calculated when knowing the difference of voltages U₄and U₁ and the difference of voltages U₁ and U₂, as well as thedifference of the respective times t₄−t₁ and t₂−t₁. The same can againbe adjusted by the sensor control and are thus considered to be known.As described above, the voltage values U₁ to U₄ are now temporarilystored on the respective sample capacitances C_(s0) to C_(s3) 254 a-254d, and the respective differences U₃−U₁, U₁−U₂ and U₄−U₁ can be formed,for example, by the CDS stage 470 connected to the sample and holdcircuit 550 and temporarily stored on memory capacitances in the CDSstage for further processing. In other words, the voltage valuesnecessitated for determining the distance to an object to be measuredcan be made available again.

FIG. 11 b shows exemplarily in two schematical time diagrams I and IItwo different options for resetting the pixel (pixel reset 229) duringthe dark period and the radiation pulse period according to embodimentsof the present invention. On a time axis, where the time t is plotted inarbitrary units, a pixel reset 229 can be performed respectively priorto the beginning of a cycle 72 including a dark period 70 a and aradiation pulse period 70 b (diagram I). For a subsequent measurementcycle, again a common pixel reset 299 can be performed for the darkperiod 70 a and the radiation pulse period 70 b. In diagram II, however,prior to the beginning of every dark 70 a or radiation pulse period 70b, a pixel reset is performed. For a subsequent measurement cycle, apixel reset can be performed again separately for every radiation pulseperiod and every dark period. This has been discussed, for example, inthe context of the embodiment in FIG. 6. The sequence and the relativeduration of the radiation pulse periods 70 a and the dark periods 70 bcan deviate from the illustrated arrangement. Hence, it is also possiblethat after a radiation pulse period several dark periods follow or thatafter one dark period several radiation pulse periods follow.Additionally, the radiation pulse periods and the dark periods do nothave be directly adjacent in time, as shown in embodiments.

FIG. 12 shows schematically the structure of an image sensor andreceiver consisting, for example, of a plurality of detection means 420arranged in columns and rows, and evaluation means 470 each associatedwith the columns, rows or pixels. The receiver or image sensor cancomprise a control logic 580, which generates, among others, therespective reset signals, control signals, and row and column selectsignals as described in the context of FIGS. 5 and 6. For this purpose,the receiver or image sensor or pixel array can have, among others, acolumn address decoder 710, a respective column multiplexer 720 or ashift register and, for example, a row address decoder 730. In responseto a select signal, the pixels of one row are connected to theevaluation means 470 associated column by column or pixel by pixel. Bythe select signal, the switches 255 are connected to the respective readlines 568 and signals are passed on to the evaluation means 470 asdescribed above. In this embodiment of the present invention, anapparatus or the detection means of an optical distance measuring deviceis connected in the form of a matrix or in an array, such that, forexample, a sensor for three-dimensional distance measurement is formed.In other embodiments, different arrangements of the pixels are possible.Generally, an evaluation means 470 can be associated with a certainarray of pixels or with every pixel individually. This means every pixelcan have an associated individual evaluation means 470, i.e., forexample, a circuit for (correlated) double sampling as described inembodiments.

FIG. 13 illustrates, in a block diagram, an embodiment of a method ofoptical distance measurement. According to one embodiment, the methodcomprises emitting 810 a radiation pulse with a pulsed radiation sourceimplemented to transmit, in a temporally contiguous radiation pulseperiod, a radiation pulse having a pulse duration t_(p) that is shorterthan the radiation pulse period, and to transmit no radiation pulse in atemporally contiguous dark period. Further, the method comprises a stepof detecting 820 different amounts of radiation with a detection meansimplement to capture reflections of the radiation pulse at an objectsurface and background radiation in two overlapping detection periodsduring the radiation pulse period, and/or to capture backgroundradiation in two overlapping detection periods during the dark period.The method further comprises determining 830 a signal depending on thedistance to be measured based on the detected amounts of radiation.

Emitting 810 can be performed, for example, such that, as describedabove, a pulsed laser beam or a pulsed LED beam is emitted. In anotherembodiment of the method for optical distance measurement, emitting 810the radiation pulse and detecting 820 the different amounts of radiationis started in two overlapping detection periods during the radiationpulse period in a temporally synchronized manner, wherein in thisembodiment a first detection period having a duration t_(int1) has thepulse duration t_(p), and a second detection period has a durationt_(int2) that is longer than t_(p) and/or, wherein detecting thedifferent amounts of radiation in two overlapping detection periodsduring the dark period starts in a temporally synchronized manner afterterminating the radiation pulse period, wherein a third detection periodagain having the duration t_(int3) can have the pulse duration t_(p),and the fourth detection period with the duration t_(int4) has aduration t_(int2) that is longer than t_(p).

Detecting 820 the different amounts of radiation with a detection meansand determining 830 a signal can be performed, for example, such thattwo charge amounts or voltage values detected in the two overlappingdetection periods depending on the detected amount of radiation aretemporarily stored in a detection means and signals depending on thedistance to be measured are determined based on the temporarily storedcharge or voltage values. In embodiments of the method for opticaldistance measurement, determining 830 a signal with a circuit forcorrelated double sampling is performed such that differential signalsare determined from the reflections of the radiation pulse at an objectand a background radiation in the two overlapping detection periodsduring the radiation pulse period and the background radiation detectedin the two overlapping detection periods during the dark period.

In a further embodiment of the present invention, in a multiple samplingmethod, electrically coupling a first sample capacitance by means of avoltage follower circuit to an output of a capacitive pixel sensorelement is performed during a first time window and electricallycoupling a second sample capacitance by means of a voltage followercircuit to the output of the capacitive pixel sensor element isperformed during a second time window. Thereby, the first and secondtime windows overlap, such that at the end of the first and second timewindows different voltage signals describing a charge or dischargeprocess of the capacitive pixel sensor element are applied to the firstand second sample capacitances. This multiple sampling method canfurther comprise a step of subtracting of the voltage signal applied tothe first sample capacitance at the end of a first period with thevoltage signal applied to the first sample capacitance at the end of asecond period, and the voltage signal applied to the second samplecapacitance at the end of the first period with the voltage signalapplied to the second sample capacitance at the end of a second period.The first period can, for example, be the radiation pulse period, andthe second period can be the dark period.

In one embodiment of the present invention, the detection means 420 isimplemented as a pixel sensor element 125 a, wherein the pixel sensoroutput is connected to the sample and hold circuit 55 o implemented inparallel via at least one buffer amplifier 134, wherein every parallelbranch of the sample and hold circuit comprises a sample switch 252 a,252 b connecting the output 134 a of the at least one buffer amplifier134 to a sample capacitance 254 a, 254 b and a transfer switch 560 a,560 b. Every transfer switch 560 a, 560 b of a parallel branch is itselfelectrically coupled to an evaluation means 470 via a further bufferamplifier 264. The sample switches 252 a and 252 b of the sample andhold circuit 550 are controlled such that they are closed in twodifferent overlapping detection periods, so that different signals of apixel sensor element are temporarily stored on every sample capacitance252 a, 252 b of a parallel branch.

The buffer amplifiers 134 and 264 can be implemented, for example, asvoltage followers in embodiments in order to buffer the photodiode orthe sample and hold circuit. The voltage followers can thus act asimpedance converter with an ideally infinitely large input resistanceand a negligible output resistance.

In other embodiments of the present invention, the evaluation means 470is implemented as a circuit for correlated double sampling. The circuitfor correlated double sampling comprises an amplifier 280, wherein theoutput of the amplifier is connected to an input 280 b via a resetswitch 282. The circuit for correlated double sampling can have first274 a and second 274 b sample capacitances connected in parallel, eachelectrically coupled to an input of the evaluation means 470 a via first572 a and second 572 b sample switches and connected to an input of theamplifier. In other embodiments, the circuit for correlated doublesampling can also have only one sample capacitance 274 and/or one switch572.

As shown in embodiments, the circuit for correlated double sampling canhave first 284 a and second 284 b feedback capacitances connected inparallel, that are connected to the input of the amplifier 280 b. Eachof the feedback capacitances is connected to a reference potential 275with its other terminal via the respective reference voltage switch 290a, b and to the output of the amplifier 280 via first 288 a and second288 b amplifier switches. In other embodiments of the present invention,the optical distance measuring device or the apparatus for doublesampling comprises a controller 580 that is implemented to close, in afirst phase A, the first sample switch 572 a of the evaluation means470, the reset switch 282 and the reference voltage switch 290 such thata first signal applied to the input of the evaluation means 470 isformed on the first sample capacitance 274 a. In a subsequent secondtime phase B, the first sample switch 572 a is opened by the controller580, the reset switch 282 remains closed, and the second sample switch572 b of the evaluation means is closed for forming a second signal onthe second sample capacitance 274 b. In a subsequent third time phase C,the controller can cause the first sample switch 572 a and the firstamplifier switch 288 a to be closed and the second sample switch 572 band the reset switch 282 to be opened in order to apply a third signalto the first sample capacitance 274 a such that a differential signal ofthe first and third signals is formed on the first feedback capacitance284 a. The controller 580 can be implemented such that in a fourth timephase D the second sample switch 572 and the second amplifier switch 288b are closed, so that a fourth signal is applied to the second samplecapacitance 274 b and a further differential signal is formed from thesecond and fourth signals on the second feedback capacitance 284 b whichis applied to the output 299.

As has been shown, for example in the context of FIG. 12, inembodiments, an optical distance measuring device can comprise aplurality of detection means 420 for detecting different amounts ofradiation arranged in rows and columns. Further, the same can comprise acontroller for controlling the rows and columns. The control logic orcontroller can be used, for example, for selecting the detection meansin one row and for selecting or controlling the shutters or switches aswell as the column address as described above. In one embodiment, anevaluation means 470, connected to the detection means 420 arranged inrows and columns via a read line 568, is associated to every column orevery row of the pixel array, such that, based on the detected amountsof radiation, signals depending on the distance of the object to bemeasured are determined. The determined signals can then be provided forfurther processing row by row or, in other embodiments, also column bycolumn or pixel by pixel. In another embodiment, an evaluation means 470can be associated with every pixel.

In embodiments of the present invention, generally, an apparatuscomprises a pixel sensor element 125 a that performs a charging ordischarging process depending on a measured quantity that can bedetected at a pixel sensor element output 134 a. Additionally, theapparatus can comprise at least one voltage follower 134, as well asfirst and second sample capacitances 252 a, 254 b as well as first andsecond switches 252 a and 252 b, connecting the first 254 a or thesecond 254 b sample capacitance in parallel to the pixel sensor elementoutput via the at least one voltage follower 134. Further, an apparatusaccording to an embodiment of the present invention can comprise acontroller implemented to control the first and second switches 252 aand 252 b such that the first switch is closed in a first time windowand the second switch is closed in a second time window, wherein thefirst time window 304 and the second time window 314 temporally overlapsuch that, at the end of the first 304 and second time window 314,different voltage signals U₁, U₂ describing the charge or dischargeprocess of the capacitive pixel sensor element 125 a are applied to thesample capacitances 254 a and 254 b.

In embodiments of the present invention, the measured quantity causing acharge or discharge process of the pixel sensor element is frequently apulsed radiation. In other embodiments of the present invention,however, other measured quantities, which generally cause a capacitivesensor element to perform a charge or discharge process, can be detectedwith apparatuses and methods for correlated double sampling described inthe embodiments.

The apparatuses or the method for correlated double sampling describedin the embodiments can also be used, for example, in other fields ofimage capturing or distance measurement.

A usage of the correlated double sampling method illustrated in thepresent invention is also possible in resistive, inductive, capacitive,piezoelectric, magnetic-field or temperature sensors. Generally, theinventive correlated double sampling method or the apparatus for doublesampling can be used in wide fields of measurement technology ormeasurement value detection.

According to a further embodiment of the present invention, the pixelsensor element 125 a can be implemented as PN diode, as photogate, asso-called charged coupled device (CCD), as PIN diode, as CCD photogate,as photonic mixer, as N well photodiode or, for example, also as pinnedphotodiode. The pixel sensor element can therefore be implemented as acapacitive pixel sensor element, wherein a charge or discharge processis performed based on a radiation interacting with the capacitive pixelsensor element or generally a measured quantity.

In another embodiment, the controller 580 can be implemented such thatthe first switch 252 a for the first time window 314 and the secondswitch 252 b for the second time window 316 are closed simultaneously atthe beginning of the charge or discharge process in dependence on themeasured quantity, wherein the duration 304 a of the first time windowis shorter than the second duration 316 a of the second time window. Theduration of the first time window can correspond, for example, to aninteraction period of a measured quantity, depending on which acapacitive pixel sensor element performs a charge or discharge process.

An evaluation means 470 can, for example, also be implemented such thatdifferential signals are determined based on two voltage signals appliedone after the other to the first 274 a and second 274 b samplecapacitances.

In one embodiment, the apparatus can have a capacitive pixel sensorelement performing a charge or discharge process in dependence on ameasured quantity that can be detected at a pixel sensor element output134 a. Additionally, the apparatus can comprise at least one voltagefollower 134, first 254 a and second 254 b sample capacitances, first252 a and second 252 b switches connecting the first or second samplecapacitance in parallel to the pixel sensor element output 134 a via theat least one voltage follower. Further, an inventive apparatus can alsohave a controller 580 implemented to control first 252 a and second 252b switches such that the first switch is closed in a first time window314 and the second switch is closed in a second time window 316, whereinthe first time window and the second time window overlap in time, suchthat at the end of the first and second time windows different voltagesignals describing the charge and discharge process of the capacitivepixel sensor element are applied to the sample capacitances. Theapparatus can further comprise an evaluation means 470 having anamplifier 280, e.g. an operational amplifier, wherein the output 280 aof the amplifier is connected to the input 280 b of the amplifier via areset switch 282. Further, the evaluation means can comprise third 274 aand fourth 274 b sample capacitances that are electrically coupled inparallel to first 254 a and second 254 b sample capacitances via third572 a and fourth 572 b switches and are coupled to the input 280 b ofthe amplifier. The evaluation means 470 can further have first 284 a andsecond 284 b feedback capacitances connected in parallel that can beconnected to the input of the amplifier and, via a reference voltageswitch 290 a,b to a reference potential 275 and, via first 288 a andsecond 288 b amplifier switches, to the output 280 a of the amplifier.Thereby, in this embodiment, the controller 580 can be implemented tocontrol the first, second, third and fourth switches as well as thereset switch, the reference voltage switch and the amplifier switchessuch that the evaluation means determines two differential voltagesignals in four successive phases. Thus, the differential voltagesignals are determined by subtraction. A distance measurement device canhave two feedback capacitances connected in parallel per pixel and canthus have double the amount of feedback capacitances as the pixel arrayof the distance measuring device has rows.

It is also possible that a double sampling system comprises a pluralityof apparatuses that have, as described above, a capacitive pixel sensorelement, at least one voltage follower, first and second samplecapacitances with first and second switches and a controller. Theplurality of these apparatuses can be arranged in rows and columns of amatrix, wherein one evaluation means 470 is associated with every row orevery column of the matrix that can be controlled with the help of acontroller and can be electrically coupled, via a read line, to thevoltage signals at the sample capacitances describing the charge ordischarge process of the capacitive pixel sensor elements.

In one embodiment, an optical distance measuring device according to thepresent invention comprises a photodiode structure having a photodiodecapacitance for accumulating charge carriers in response toelectromagnetic radiation, two readout capacitances, a reset means forresetting the readout capacitances by applying a predetermined voltageto the readout capacitances, a switching means for connecting thephotodiode structure to the readout capacitances to transfer theaccumulated charge carriers during two overlapping detection periodsduring a transfer phase determined by opening shutters 252 a or 252 b tothe readout capacitances, and for separating the photodiode structurefrom the readout capacitances after the accumulation phases or aftertheir respective overlapping detection periods in the radiation pulseperiod and in the dark period, and a readout or evaluation means forreading out the readout capacitances, wherein the readout means isimplemented to read out the readout capacitance for a first time duringthe accumulation phase in the radiation pulse period and a second timeduring an accumulation phase in the dark period to obtain first andsecond readout values from the first and second overlapping detectionperiods, and to combine the values to obtain two readout results.

The present invention can enable noise reduction if readout capacitancesare associated with a photodiode structure with a photodiode capacitanceseparated from the photodiode structure by a switch means and if areadout or evaluation means is provided which, for obtaining readoutresults, does not read out a readout value from the readout capacitiesonly after the accumulation phase in the radiation pulse period, butalso once after the accumulation phase in the dark period to combine thetwo readout values, such as, e.g., to calculate the difference such thatthe reset noise that can arise when resetting the readout capacitancescan be reduced or eliminated from the readout results.

According to an embodiment of the present invention, the photodiodestructure can be formed of a pinned photodiode whose space charge regioncan be depleted, which has in particular the advantage that, in a pinnedphotodiode, the light-sensitive p-n-transition is not covered by a metalelectrode and is close to the surface, so that a pinned photodiode showsa higher sensitivity.

According to a further embodiment of the present invention, a firstreadout of the first two readout values takes place in a radiation pulseperiod immediately after resetting the readout capacitances, wherein thesecond readout takes place after the end of the accumulation phase or ina subsequent transfer phase in the dark period. For this purpose, thevoltage state of a readout capacitance can be read out at any time suchthat the state of the readout capacitance in the readout process doesnot change. Additionally, the read-out voltage states of the readout orsample capacitances can be temporarily stored in an analog or also in adigitalized manner. Thereby, the portion of the reset noise of the finalreadout result is corrected in that the temporarily stored readoutvalues are subtracted from each other at the end of a complete readoutcycle, so that the contribution of the reset noise is eliminated or atleast reduced in the final readout result.

In a further embodiment of the present invention, capturing a sequenceof several subsequent light pulses is enabled without having to triggera reset event of the readout capacitance after every light pulse. Forthis purpose, at the beginning of the charge accumulation of a firstaccumulation phase in the radiation pulse period, the readoutcapacitances are reset, immediately after that the state of the readoutcapacitances will be read out and stored for the first time. Afterterminating the first charge accumulation phase, the accumulated chargeis transferred from the photodiode to the readout capacitances. Withouthaving to reset the readout capacitances, the photodiode can start a newaccumulation cycle in the dark period, since its space charge region hasbeen restored by the depletion due to the reset. At the end of the firstaccumulation cycle, transfer of the photo charges to the readoutcapacitance takes place again. This process can basically be repeated asoften as desired, wherein it has to be ensured that the prechargedreadout or sample capacitance is further discharged in every chargetransfer by the accumulated charges in the photodiode, so that themaximum number of accumulation phases to be captured results from thesize of the readout capacitance and the reset potential. This is calledfull well capacity. At the end of the last accumulation phase, thevoltage state of the readout capacitance is read out a second time andalso temporarily stored, the final readout result results by calculatingthe difference of the second readout result and the first readout resultand is again freed from the noise portion. An optical distance measuringdevice operated in that manner contributes to a significant improvementof the signal/noise ratio, and can be realized with a standard CMOSstructure. Several subsequent accumulation phases become possiblewithout having to trigger a reset event or pixel reset after everysingle accumulation phase, which would deteriorate the signal/noiseratio again due to the unavoidable noise contribution.

In one embodiment, the pixel sensor element can have a pinnedphotodiode. The same can operate as follows. Prior to distancemeasurement, an n well of the pinned photodiode is depleted, so that, atthe p-n-transition, a so-called “pin potential” U_(PD)=U_(pin) appearsacross the barrier layer capacitance. For this purpose, first, thereadout or sample capacitances are set to the potential of the supply orreset voltage V_(reset) by means of a reset signal. Then, the actualdepletion of the n well of the pinned photodiode is performed by closingthe switch 252 a,b, whereby charges still stored on the photodiodecapacitance 132 flow off towards the readout capacitances. With thefalling edge of a signal for closing the shutter 252 a a chargeaccumulation phase starts, in which the photogenerated charge carriersare accumulated in the space charge region of the pinned photodiode andthe potential at the photodiode capacitance decreases from the startpotential U_(pin) proportionately to the detected amount of light. Afteropening switches 252 a,b, i.e. after terminating the integration,voltages proportional to the photogenerated charge carriers are appliedto the readout capacitances.

It should also be noted that, depending on the circumstances, theinventive method can also be implemented in software. Implementation canalso be performed on a digital memory medium, in particular a disc, a CDor a DVD having electronically readable memory signals that cancooperate with a programmable computer system and/or a microcontrollersuch that the respective method is performed.

Generally, the invention hence also consists in a computer programproduct with a program code for performing the inventive method storedon a machine-readable carrier when the computer program product runs ona computer and/or microcontroller and/or a digital signal processor DSP.In other words, the invention can be realized as a computer programhaving a program code for performing the method when the computerprogram runs on a computer and/or a microcontroller.

Generally, the methods or apparatuses presented in this invention canalso be applied in fields of application outside 3D distance measurementor 3D image capturing.

The detection principle presented in embodiments does not useconventional serial 3D measurement value capturing, but a parallelmethod which has advantages, as has been described above. For example,the laser energy necessitated for the measurement can be reduced by halfand the measurement speed and object resolution can be improved, since alaser recovery time or a recovery time for a pulsed radiation source,that can be several milliseconds depending on the emitted power, doesnot have to be maintained. This opens up new fields of application forthree-dimensional distance measurement sensor technology and imagecapturing with pulsed light in general. For example, for high-speedcameras operating according to the method described herein, it ispossible to follow the trajectory of flying objects with high speed,such as rockets or projectiles. Also, it is possible for distancecameras in vehicles to be substantially more robust and hence to bebetter able to reliably keep a distance to another vehicle or, forexample, a pedestrian. Existing 3D measurement systems operating, forexample, with laser light in the spectral range of 900 nanometers wouldnecessitate a much higher pulse energy since two pulses are necessitatedfor reliably determining the distance. It is also possible that, whenmaintaining the used laser energy of the standard method, alternativelythe laser power associated with the width of the integration period canbe increased. This increases the signal/noise ratio between the laserand the background light and hence the measurement resolution. In otherwords, the laser pulse performance can be increased and hence themeasurement resolution can be improved, wherein the overall laser energyused for the measurement can be maintained. Compared to conventionalexisting applications, the improvement in the signal/noise ratio isgiven approximately by the ratio of the laser energies. In an 0.35micrometer standard CMOS production process, three-dimensional arrays ormatrices having approximately 2,000 pixels can, for example, beintegrated with an edge length of 100 to 200 micrometers.

According to another aspect of the present invention, the describedmethod or the apparatus can also be used in two-dimensional CMOS imagesensors. The double-shutter method, as described above, enablesdifferential image applications in the two-dimensional range operatingwith active pulsed lighting.

In 3D pictures, reflectance correction can be realized on a system levelby means of double-shutter methods, whereby a differentiation is madebetween correction calculations “on-chip”, i.e. on the image sensor, and“off-chip”, i.e. not on the image sensor (software, firmware). Thisprocedure can also be applied to the double-shutter method as presentedin this invention. Therewith, it is possible, in addition to the simplesubtractive background light correction, to realize also the reflectancecorrection either close to the hardware on the sensor chip and/or in asystem comprising the sensor chip, in a computer program or software.

In embodiments, digitalization of the detected voltage signals can takeplace directly after the sample and hold circuit 550 and a subsequentsubtraction and/or division of these digital values is also performed“on-chip”. Also, these digitalized voltage values can be subtractedand/or divided externally, for example in the camera system or in acomputer, in order to perform the distance calculation. The subtractionof the voltage values detected by the sample and hold circuit can,therefore, be performed in other embodiments in a different way to thatof the CDS stage, namely, for example, digitally “on-chip” orexternally.

In another embodiment, the differential voltage values at the output ofthe evaluation means 470 or the CDS stage can be digitalized “on-chip”,and correspondingly an “on-chip” division can be performed after the CDSstage to determine the distance value. The division of these digitalvalues can also be performed again externally, for example in a camerasystem or in a computer.

In embodiments, the radiation pulse can have a variable starting pointwith regard to the integration windows, i.e. a variable time offset. Theintegration windows or the detection periods can, therefore, start withan adjustable known time offset or phase-locked to each other.

Between the shutter signals for opening and closing the time windows fordetecting the voltage signals and the signal for starting the radiationpulse a delay line can, for example, be inserted in a functional mannerin order to allow time-variable and, hence, distance-variablephase-locked starting of the time windows and the radiation pulse. If,for example, an object is far away, a delay line having an adjustabledefined duration can compensate this respective runtime until the startof the time window. The detection periods for detecting the amounts ofradiation, i.e. the integration windows, can have a variable integrationtime that can, for example, be adjusted and controlled by a controllerand are, hence, known.

As shown in embodiments, the voltage values detected and temporarilystored by the sample and hold circuit can be serially subtracted in theCDS stage, or, if the CDS stage comprises a second operationalamplifier, also be completely processed in parallel. As has been shownin embodiments, the CDS stage can also have, for example, only oneoperational amplifier and the respective sample capacitances 274 andfeedback capacitances 284 can still be implemented in parallel. Thisenables temporary storage of the voltage values detected in overlappingdetection periods in the CDS stage for the subtraction of the detectedvoltage values performed in the CDS stage.

In embodiments of the present invention, an evaluation means or a CDSstage 470 can be associated with a row or column of a pixel array of asensor 420, or every pixel has an associated CDS stage.

It should be noted that the buffer amplifiers 134 and 264 in theembodiments can also be amplifiers with a buffer effect having anamplification of more than x1. The buffer amplifiers 134 and 264therefore do not necessarily have to be implemented as voltage followersor source followers. For an improved signal/noise ratio it can, forexample, be advantageous to amplify the voltage signal of the pixelelement 125 a as early as possible in the signal path. This can beachieved by using a buffer amplifier as a buffer 134 allowing anamplification of more than x1 and a respective buffering of the voltageat the photodiode. The buffer 134 and 264 can, for example, be anoperational amplifier having an amplification of more than x1 and anappropriate buffer effect.

In comparison to conventional methods, the methods described in theembodiments can achieve a significantly higher correlation of distanceand reflectance measurements by multiple accumulation. Thereby, forexample, fast-moving objects can be detected better and reduced“blurring” of the object can be obtained.

While this invention has been described in terms of several advantageousembodiments, there are alterations, permutations, and equivalents whichfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing the methods andcompositions of the present invention. It is therefore intended that thefollowing appended claims be interpreted as including all suchalterations, permutations, and equivalents as fall within the truespirit and scope of the present invention.

1. An optical distance measuring device comprising: a pulsed radiationsource implemented to transmit, in a temporally contiguous radiationpulse period, a radiation pulse with a pulse duration that is shorterthan the radiation pulse period, and to transmit no radiation pulse in atemporally contiguous dark period; a detector for detecting differentamounts of radiation in two overlapping detection periods during theradiation pulse period to capture reflections of the radiation pulse atthe object surface and a background radiation and/or in two overlappingdetection periods during the dark period to capture a backgroundradiation; and an evaluator determining a signal depending on a distanceof the optical distance measuring device to an object, based on thedetected amounts of radiation.
 2. The optical distance measuring deviceaccording to claim 1, wherein the detector is implemented such that thetwo overlapping detection periods during the radiation pulse periodstart phase-locked with the radiation pulse, wherein a first of the twodetection periods during the radiation pulse period comprises a firstduration and a second of the two detection periods during the radiationpulse period comprises a second duration, which is longer than a pulseduration of the radiation pulse, and such that the two overlappingdetection periods during the dark period start phase-locked after thetermination of the radiation pulse period, wherein a first of the twodetection periods during the dark period comprises a third duration,which corresponds to the first duration, and a second of the twodetection periods during the dark period comprises a fourth duration,which corresponds to the second duration.
 3. The optical distancemeasuring device according to claim 1, wherein the detector isimplemented such that the two overlapping detection periods during theradiation pulse period start phase-locked with the radiation pulse ofthe pulsed radiation source, wherein a first of the two detectionperiods during the radiation pulse period comprises a first duration anda second of the two detection periods during the radiation pulse periodcomprises a second duration, which is longer than a pulse duration ofthe radiation pulse, and such that the two overlapping detection periodsduring the dark period start prior to or after the termination of theradiation pulse period, wherein a first of the two detection periodsduring the dark period comprises a third duration, which differs fromthe first duration, and a second of the two detection periods during thedark period comprises a fourth duration that differs from the secondduration.
 4. The optical distance measuring device according to claim 1,wherein the detector is implemented such that the two overlappingdetection periods during the radiation pulse period comprise a variabletime offset with respect to the radiation pulse of the pulsed radiationsource.
 5. The optical distance measuring device according to claim 1,wherein the evaluator is implemented such that the detected signal is adifferential signal generated by subtracting temporarily stored signalsdescribing the different amounts of radiation detected during theradiation pulse period in the two overlapping detection periods, andsignals describing the different amounts of radiation detected duringthe dark period in the two overlapping detection periods.
 6. The opticaldistance measuring device according to claim 1, wherein the pulsedradiation source is a laser or an LED emitting electromagneticradiations in the ultraviolet (UV), visible (VIS), infrared (IR) orfar-infrared (FIR) spectral range.
 7. The optical distance measuringdevice according to claim 1, wherein the detector is a pixel sensorelement providing signals at a pixel sensor element output in dependenceon the detected amounts of radiation, wherein the pixel sensor elementoutput is connected to the sample and hold circuit implemented inparallel via at least one buffer amplifier, wherein every parallelbranch of the sample and hold circuit comprises a sample switchconnecting the output of the at least one buffer amplifier to a samplecapacitance and a transfer switch, wherein, by the transfer switch,every parallel branch of the sample and hold circuit is connectable tothe evaluator, and wherein the sample switch in every parallel branch isimplemented such as to be closed in two overlapping detection periods,so that after terminating the overlapping detection periods a differentsignal of the pixel sensor element output is temporarily stored on everysample capacitance of the parallel branch.
 8. The optical distancemeasuring device according to claim 1, wherein the detector is a pixelsensor element providing signals at a pixel sensor element output independence on the detected amounts of radiation, wherein the pixelsensor element output is connected to a selection switch of the sampleand hold circuit via at least one buffer amplifier, wherein the sampleand hold circuit comprises at least two sample capacitances that can becoupled to the input and output of the sample and hold circuit via onesample switch each, and wherein the sample switch is implemented such asto be closed in two temporally different detection periods, so thatafter terminating the detection periods a different signal of the pixelsensor element output is temporarily stored on each of the at least twosample capacitances.
 9. The optical distance measuring device accordingto claim 8, wherein the buffer amplifier is implemented as a voltagefollower or as an operational amplifier for amplification.
 10. Theoptical distance measuring device according to claim 1, wherein thedetector comprises a PN photodiode, a PIN photodiode, a pinnedphotodiode, a photogate, an N-well photodiode, a CCD photogate or aphotonic mixer that is implemented to detect the different amounts ofradiation in two overlapping detection periods.
 11. The optical distancemeasuring device according to claim 1, wherein the evaluator comprises acircuit for correlated double sampling, comprising: an amplifier,wherein the output of the amplifier can be connected to an input of theamplifier via a reset switch; a sample capacitance that can be connectedto an input of the evaluator and an input of the amplifier via a sampleswitch; first and second feedback capacitances connected in parallelthat can each be connected to an input of the amplifier and to areference potential via a reference voltage switch, and to the output ofthe amplifier via first and second amplifier switches; a controllerimplemented to switch the reset switch, the sample switch, the referencevoltage switch and the first and second amplifier switches such as tosubtract the signals applied to the input of the evaluator from eachother in a serial sequence and to provide the respective differentialsignals at the output.
 12. The optical distance measuring deviceaccording to claim 1 comprising a plurality of detectors for detectingdifferent amounts of radiation and a controller for controlling a pixelarray, wherein an evaluator that can be connected to the detectors via aread line is associated with every pixel array or every pixel, such thatthe evaluators determine, based on the amount of radiation detected withthe detectors, signals depending on the distance of the optical distancemeasuring device to an object, and wherein the signals are provided to apixel array or pixel by pixel for further processing.
 13. The opticaldistance measuring device according to claim 1, wherein the signalprovided by the evaluator can be digitalized “on-chip”, and a subsequentdivision of the digital values for determining the distance can beperformed “on-chip”.
 14. An apparatus comprising: a capacitive pixelsensor element subjected to a charge or discharge process in dependenceon a measured quantity that can be detected at a pixel sensor elementoutput; at least one buffer amplifier; first and second samplecapacitances; first and second switches, via which the first or thesecond sample capacitance can be connected to the pixel sensor elementoutput via the at least one buffer amplifier; a controller that isimplemented to control the first and second switches such that the firstswitch is closed in a first time window and the second switch is closedin a second time window, wherein the first time window and the secondtime window temporally overlap such that, at the end of the first andsecond time windows, different voltage signals describing the charge ordischarge process of the capacitive pixel sensor element are applied tothe first and second sample capacitances.
 15. The apparatus according toclaim 14, wherein the voltage signals describing the charge or dischargeprocess of the capacitive pixel sensor element can be digitalized“on-chip” at the first and second sample capacitances, and a subtractionand/or division of the digitalized voltage signals can be performed“on-chip” for determining a distance value to an object.
 16. Theapparatus according to claim 14, wherein the measured quantity is apulsed radiation.
 17. The apparatus according to claim 14, wherein thepixel sensor element is implemented as PN diode, photogate,charged-coupled device (CCD), PIN diode, CCD photogate, photonic mixer,N-well photodiode or as pinned photodiode.
 18. The apparatus accordingto claim 14, wherein the pixel sensor element is implemented such thatthe charge or discharge process is performed based on a radiationinteracting with the capacitive pixel sensor element.
 19. The apparatusaccording to claim 14, wherein the controller is implemented such thatthe first switch is closed for the first time window and the secondswitch for the second window with an adjustable known time offset at thebegin of the charge or discharge process in dependence on the measuredquantity, wherein the duration of the first time window is shorter thanthe duration of the second time window.
 20. The apparatus according toclaim 14, wherein the first time window is closed during an interactionperiod of the measured quantity in dependence on which the capacitivepixel sensor element performs the charge or discharge process.
 21. Theapparatus according to claim 14, further comprising an evaluatordetermining a differential voltage signal based on voltage signalsapplied successively to the first and second sample capacitances. 22.The apparatus according to claim 21, wherein the evaluator furthercomprises: an amplifier, wherein the output of the amplifier can beconnected to an input via a reset switch; a sample capacitance that canbe electrically coupled to the first and second sample capacitances viaa switch and that can be connected to the input of the amplifier; firstand second feedback capacitances connected in parallel that can becoupled to the input of the amplifier and can be connected to areference potential via a reference voltage switch and to the output ofthe amplifier via first and second amplifier switches; and wherein thecontroller is implemented to control the evaluator such thatdifferential voltage signals of the voltage signals applied to the inputare available at the output of the evaluator in successive time phases.23. A double sampling system comprising a plurality of apparatuses, theapparatuses comprising: a capacitive pixel sensor element subjected to acharge or discharge process in dependence on a measured quantity thatcan be detected at a pixel sensor element output; at least one bufferamplifier; first and second sample capacitances; first and secondswitches, via which the first or the second sample capacitance can beconnected to the pixel sensor element output via the at least one bufferamplifier; a controller that is implemented to control the first andsecond switches such that the first switch is closed in a first timewindow and the second switch is closed in a second time window, whereinthe first time window and the second time window temporally overlap suchthat, at the end of the first and second time windows, different voltagesignals describing the charge or discharge process of the capacitivepixel sensor element are applied to the first and second samplecapacitances; wherein the apparatuses are arranged in arrays, wherein anevaluator that can be coupled, with the help of a controller forcontrolling the arrays or the individual apparatuses via a select switchand a read line, to the voltage signals of the sample capacitancesdescribing the charge or discharge process of the capacitive pixelsensor element is associated with every array or every single apparatus.24. A method for optical distance measurement, comprising: emitting aradiation pulse with a pulsed radiation source implemented to transmit,in a temporally contiguous radiation pulse period, a radiation pulsewith a pulse duration t_(p) that is shorter than the radiation pulseperiod, and to transmit no radiation pulse in a temporally contiguousdark period; detecting different amounts of radiation with a detectorthat is implemented to capture reflections of the radiation pulse at anobject surface and background radiation in two overlapping detectionperiods during the radiation pulse period and/or to capture backgroundradiation in two overlapping detection periods during the dark period;and determining a signal depending on the distance to be measured basedon the detected amounts of radiation.
 25. The method for opticaldistance measurement according to claim 24, wherein emitting a radiationpulse is performed such that a pulsed laser beam or LED beam is emitted.26. The method according to claim 24, wherein emitting the radiationpulse with a pulse duration t_(p) and detecting the different amounts ofradiation in two overlapping detection periods during the radiationpulse period starts phase-locked with emitting the radiation pulse,wherein a first of the two detection periods comprises the pulseduration t_(p) and the second of the two detection periods comprises aduration t_(int2) that is longer than t_(p) and/or wherein detecting thedifferent amounts of radiation in two detection periods during the darkperiod is performed prior to or after the radiation pulse period. 27.The method according to claim 24, wherein detecting different amounts ofradiation with a detector is performed such that two amounts of chargeor voltage values depending on the amount of radiation detected in thetwo overlapping detection periods are temporarily stored in the detectorand signals depending on a distance to an object to be measured aredetermined based on the temporarily stored amounts of charge or voltagevalues.
 28. The method according to claim 24, wherein determiningsignals with a circuit for correlated double sampling is performed suchthat differential signals are determined from the reflections andbackground radiations detected in the two overlapping detection periodsduring the radiation pulse period, and from the background radiationsdetected in the two detection periods during the dark period.
 29. Themethod according to claim 24, wherein detecting is performed such thatthe background radiation in two detection periods in a dark period isdetected simultaneously for all detectors of a sensor prior to theradiation pulse period.
 30. The method according to claim 29, whereinthe background radiation is extrapolated based on the detectedbackground radiation in the two detection periods during the darkperiod, and a distance to the object to be measured is determined basedon the extrapolation values.
 31. A multiple sampling method, comprising:electrically coupling a first sample capacitance to an output of acapacitive pixel sensor element via a buffer amplifier during a firsttime window with a first duration, and electrically coupling a secondsample capacitance to the output of the capacitive pixel sensor elementvia a buffer amplifier during a second time window with a secondduration, wherein the first and the second time window temporallyoverlap, such that at the end of the first and second time windowsdifferent voltage signals describing a charge or discharge process ofthe capacitive pixel sensor element are applied to the first and secondsample capacitances.
 32. The multiple sampling method according to claim30, further comprising a step of calculating the difference of thevoltage signal applied to the first sample capacitance at the end of afirst time window with a first duration and the voltage signal appliedto the first sample capacitance at the end of a third time window with afirst duration, and calculating a difference of a voltage signal appliedto the second sample capacitance at the end of a second time window witha second duration and the voltage signal applied to the second samplecapacitance at the end of a fourth time window with a second duration,wherein the first and second as well as the third and fourth timewindows temporally overlap.
 33. A computer program comprising a programcode for performing a method for optical distance measurement, themethod comprising: emitting a radiation pulse with a pulsed radiationsource implemented to transmit, in a temporally contiguous radiationpulse period, a radiation pulse with a pulse duration t_(p) that isshorter than the radiation pulse period, and to transmit no radiationpulse in a temporally contiguous dark period; detecting differentamounts of radiation with a detector that is implemented to capturereflections of the radiation pulse at an object surface and backgroundradiation in two overlapping detection periods during the radiationpulse period and/or to capture background radiation in two overlappingdetection periods during the dark period; and determining a signaldepending on the distance to be measured based on the detected amountsof radiation, wherein the program code is performed on a computer, amicrocontroller or a digital signal processor.